Welcome, we're happy to have you with us!Well, as you probably know by now, there are no verses that directly cover smoking in the Bible.However, we know from several scriptures that our body is a temple:I Corinthians 6:19-20
What? know ye not that your body is the temple of the Holy Ghost which is in you, which ye have of God, and ye are not your own?A very similar statement is found in I Corinthians 3:16 and then is followed by this:I Corinthians 3:18
If any man defile the temple of God, him shall God destroy; for the temple of God is holy, which temple ye are.God certainly isn't going to destroy your friends for what they do but I think the answer is that yes, it is a sin when not controlled. Our bodies and souls both belong to God. The problem with smoking is that it becomes addictive and something done neither in moderation nor in health. It does ultimately harm the body and those around them as well. Most smokers know and will admit that this is a bad habit for them.However, do remember that: Romans 3:23
For all have sinned, and come short of the glory of God; I'm not sure if you're going to talk with your friends or what, but it would be best for them not to smoke. The Bible is a very good health manual and has wonder wisdom for issues like this. Keep in mind that it's ultimately up to your friends, though.Hope that answers your question and as always, anyone and everyone is free to reply.