“…But other than it being illegal, I can’t really find anything wrong with it. And the fact that it hurts your body, but so does alcohol and tobacco, but I don’t think they are wrong to use.”
It is illegal, and by following the law you are not breaking God’s moral law… that is all the reason any Christian needs.  Romans 13 makes this abundantly clear.  It is an acceptable role of authority to create laws about safety, etc. for its people. If the obedience of a law puts one in direct conflict with what we know to be morally right, as in Paul & Silas being commanded to stop teaching about Jesus, it is right for us to disobey that law.  Otherwise, it is a key tenet of Christianity to obey our authorities.  It is improper to ask a question like this with “… but other than being illegal…” because that is a central issue here!  Unless the laws of man cause us to disobey the laws of God, we obey them whether we agree with them or not.  I feel safe in saying that refraining from using marijuana does not breach the laws of heaven.
Now, would it be wrong for a person to absorb marijuana if it were legal?  I don’t know that I could make a concrete case for that biblically, since marijuana is not discussed in scripture.  I will tell you that using the passage in Genesis (I assume they mean Gen 1:29 “Then God said, “Behold, I have given you every plant yielding seed that is on the surface of all the earth, and every tree which has fruit yielding seed; it shall be food for you…” (NASU) as a defense for smoking marijuana is ridiculous.  Pinecones are seeds… and Cyanide is also found in seeds, but I don’t think this passage encourages the usage of cyanide.  Do they take all of God’s words to Adam and apply them to themselves?  Are they gardeners?  That is terrible usage of a passage to defend one’s illegal and possibly immoral (I am getting to that) behavior and is probably akin to a serious abuse of God’s word in order to defend one’s sin.  I personally would not be interested in answering for that kind of junk.  To your “friends” I would say if you are going to do something they know is wrong, they don’t need to twist scripture to defend their behavior. 
There is one thing that MJ does that concerns me the most.  When people are on it, they describe something similar to:  if they were in a bath that was splashing around, the MJ “stills” the water and splashes.  This sounds positive, but the problem comes in when MJ doesnt know what to “still” and what not to.  Soon, sports, grades, relationships, and other things that were once enjoyed, or at least motivating, become less signficant and interesting.  Perhaps most insidious of all, the user seems totally unable to see this trend, no matter how obvious it is to everyone else!
Back to what the Bible does say… it does not mention marijuana in particular, but it is interesting that the Greek word used for “witchcraft” in Gal 5:20 is “farmakeia”  from which we get the  word “pharmacy.”  It probably references how people used drugs to create a “spiritual” sense to make themselves feel close to the spirits.  It is clearly condemned here… that is one direction to go… but not conclusive in any way to our discussion, of course…
but another one is the concept of drunkenness, which scripture also condemns.  Eph 5:8 commands one not to get drunk with wine (I don’t think it is saying it is ok to get drunk with other things, just not wine, by the way).  In fact, the Greek word for “drunk” here and in other passages is methe, and refers to any kind of “intoxication.”   I think it doesn’t take a scholar to see that it would probably be accurate to apply drunkenness/intoxication to “high”… especially when you consider the phrase “leads to dissipation.”  Dissipation is a reference to stealing life from you – like something melting away.  Know anyone who does marijuana?  You have probably seen their lives seem to slip away from them over time.   It can be very painful for family members and friends who love a person to watch their loved one’s life dissapating.  Most of us want great things for our children, but marijuana will slowly “still” virtually all areas of life.
Remember the feeling of not caring about anything that comes with smoking it?  I will tell you that I personally, having watched what marijuana does to families and to the lives of individuals, I have learned to hate it.  Here is what it does:  it sells itself as a simple, “natural” (how illogical a reason is that?  Snake bites are natural, and cyanide is natural too, as is gravity when you fall from a ten story building.  Natural does not always equal positive or good) pleasure, but it is not.  It is a thief of life and is usually used by people to medicate their anger, pain, and their need to grow and change.  Anxiety and challenges are normal parts of life that help us grow… MJ, oever time, steals that motivation from us.
Often, people smoke as a part of their peer group friendships.  I know that many of these friendships are very real, and some of the other users can be great people.  However, so long as they are using, my recommendation is to flee these “friends.”  Proverbs tells us how important it is to have wise and good people around us.  Are you short for people living the life of godly men and women?  I think the Christian life is meant to offer something better than any kind of false feeling or numbed life…  take a look at Ephesians 5:18.
Eph 5:18
And do not get drunk with wine, for that is dissipation, but be filled with the Spirit…”
How about spending time being filled with the Spirit rather than filled with some other intoxicant?
So, illegal (which ends the argument), connected to sorcery, and drunkenness.  Chances are that your using friends have made their decision who they are going to serve and your words will not change them.  They aren’t looking for a reason to change, plus MJ is a parasite which sucks the desire to live life in any abundant way anyway.  If they cannot walk away, you had better, or they will probably have you back in it soon – you cannot serve two gods and still be friends long… MJ may not be physiologically addictive, but it certainly can become psychologically addictive… but its ability to “still” things in life is what makes it most dangerous, in my book. 
Maybe it would be helpful to hear about how the power of hashish to still our sense of morality was recognized and taken advantage of even as far back as the Crusades!  One branch of the Muslim faith was not very powerful militarily so they devoted themselves to clandestine murder for hire as a way to keep some power in the region.  Part of their ritual for preparation for a killing would be to calm themselves and dull their sense of morality by smoking hashish.  Over time they became known as “hashishians.”  Today we have the word “assassin.” 
Finally, as a therapist, I watch marijuana slowly steal motivation, drive and passion from people’s lives.  Slowly, over time (depending on frequency of usage), one April 20^th at a time, users lose interest in family, school, career, relationships, God, everything.  Watch someone who has been a consistent user for some time and see if you can see, because I am certain that they cannot see it – they never can… and neither can you, if you are the user.
Clear enough?
Chris