I have many different groups of friends. I like to keep the groups of friends seperate for personal reasons. How can I control this?

Sometimes I’ll have plans with a friend to just hang out, see a movie, go to the bar, etc. But then that night another friend from a separate group will call and ask what I’m up to. I’ll usually be honest with them, and tell them that I have plans to hang out with a friend and maybe we can do something the next night. I think they take offense though that I did not include/invite them to come along with whatever I was doing.

The thing is, instead of having one big group of friends, I have many different friends from many separate groups. I like to keep it that way – and usually each friend (or small group of friends) from each group is familiar with completely different sides of me and we all have different things in common. I intend to keep it this way.

But I hate offending people. How can I tastefully and maturely let friends know that they are not invited to hang out with me and my other friends, and without hurting anyone’s feelings?