I would ask them if they wanted to go bowling or play mini golf instead of smoking or doing something else fun and see how much they really enjoy your company.

Id bet they are only using you for your material possesions, cause that's how it happened to me. lol

Or if you really want to be devious, invite another much prettier girl over along with the other two for a smoking session and pay no attention to the usual two that come over, lol, only if you want to mess with them.

The Beef