This one is probably the least probable to happen but I've seen it happen. Not to me, but to someone else. I was ashamed, I must admit. I almost wanted to invite the person that was being left out just for common courtesy, but she somehow knew she was purposely being excluded. 

Come on, it's understandable that not everyone takes part in your inner circle, but do you seriously have to be rude about it? Why can't you wait to make more plans after this person leaves? It hurts, you know. It makes you feel unwanted, unappreciated, boring. 

If you want to make plans, assume that everyone that's listening is invited. If that's not your plan, then don't plan just yet or make a WhatsApp group later that day. 

In another case, perhaps you are part of the inner circle, but for any reason you almost always refuse their offers to hang out. Then people start to leave you out, or assume you're busy. In this scenario, it's common to find your friends making plans and saying "We won't even count you, you probably won't go anyway." I'm sorry to say this, but that's your fault. If you don't make time for friends, then your friends won't make time for you and move on without you :(