As your young child learns to play with others, she may need your help to learn what behaviour is acceptable. If you see that your child is upset when playing, encourage her to put all her emotions, particularly her frustrations, into words. Try to identify with your child's feelings, but let her know that there are still certain ways that one should behave in such a situation. "You want to play with that puzzle, but Jason has it now. Even though you really, badly want it, you will have to wait."
If there's been a problem with another child, help your child see the other child's point of view, and talk about possible solutions to the problem. "You grabbed Jason's puzzle and now he is crying. You made him feel badly. You will have to give the puzzle back, and wait until Jason is finished." You will need to be a good role model, as your child will be watching you to learn social skills. You will need to avoid reactions like rudeness or impatience. Little children watch adults all the time, and copy our worst, as well as our best behaviour.
Making friends works best if you let your child choose when he wants to play and whom he wants to play with. Children don't necessarily become friends with each other just because their parents are friends or relatives. Learning to get along with others takes time, so don't push your child to play with others.
If your child is having difficulty becoming part of the group or getting along with others, watch him, and see if there are ways you can help him join in. Sit on the sidelines with him and discuss what is happening in the room, where he would like to play and how he could join in to be accepted. Or give your child a toy that will fit in with the group's play, in order to help him join the group.