Great question!! My daughter is starting first grade this year at a public school after being at a small kindergarten. I am so nervous for her and expect this to come up so I will read the responses carefully for ideas.
We have had very minor hints of this in the past, and here is what I have done/said on various occasions and probably not this clearly.
First, it is okay to not always want to play with the same people all the time or the same games everytime. If a friend doesn't feel like playing your game, then you can't make them. And sometimes you feel like playing with a different friend, right? If someone doesn't want to play with you, it doesnn't mean they don't like you or that you did somehting wrong. It is just how they feel at that moment. It would be nicer if they included you, but it doesn't mean they aren't your friend.
Second, I expect YOU to include everyone in your games unless they aren't being kind to you and the group. Kindness means taking turns, letting everyone speak, and generally not being a bully.
Third, some kids aren't good at using their words to express themselves beacuse they are still learning so you need to be patient and use your words to help the situation. Ask someone if "shat's wrong?" if you feel like there is a problem and maybe you can help figure out a solution. Or tell them, "I want to play together. Can we compromise?"
Some kids, aren't encouraged to talk as nicely as you are. It doesn't mean they are bad, it just means they aren't good about thinking about how others feel So you need to tell someone when they hurt your feelings and tell them it is not okay to be hurtful.
Finally, if someone is treating you badly repeatedly, you need to remove yourself. Not forever, but just until everyone cools off. Try again another day. Ask other friends if you can play with them instead.
Easier said than doen, huh?
By the way, to echo some of the other comments, I once heard a reallly good coping strategy for girls is to make sure they have lot sof sets of friends. School friends + soccer friends + art friends + neighborhood friends etc.. That way, they never have too much invested in any one group of people. It says, "You act you don't like me, but that's your problem. I have lots of friends."