I think firstly you need to decide if you are comfortable having their friends come to stay in your home. It's not clear from your post is this every weekend? How many friends want to stay. @Carmen802
I am a homeshare host and have never had this request from a guest in six years of hosting.
If it was for one weekend and it was one additional guest I would consider it, but not for multiple guests every weekend.
I think it's really cheekly to tell you, you shouldn't for charge additional guests because they are choosing to stay elsewhere sometimes - their stuff is obviously still there and you can't re-let to anyone else so there is no reason to take this into account.
I would decline their request on this basis. @Carmen802