I'm in my late 20s and when I hang with my friends who smoke weed they know I don't smoke. When I am with them they never invite me to smoke, and even purposefully set aside a separate room to do smoke so I don't have to smell it as much. I didn't ask them to do this, they are just courteous. It would be really rude for them to keep asking me or pressuring me even after they knew I don't smoke, and if they did that I would just consider them immature and find a different group.

If you are a teenager then it is harder. Friends are hard in general, and often you don't have the liberty of expecting that level of respect. They aren't mature or polite enough to respect your decisions yet because they aren't secure in their own. Getting you to participate would validate their decisions, so they pressure you. If you are this age, I'm sorry. It's a great time, but also hard. I don't have any advice except to hang in there and it does get way easier once everyone grows up a bit - and once you get the freedom to refuse to associate with those who don't.