If you're arranging a playdate or playgroup for your 2-year-old, don't agonize over the list of participants. Children this age are still fairly flexible about playmates. By 3, though, your toddler will likely have begun to voice her preferences, and you can try to accommodate her.
If a playmate displays bullying behavior, use the incident as a teaching tool. After the gathering, discuss the behavior with your child and help her to understand why the behavior is not appropriate. Unless the situation is downright dangerous, however, don't pull your child from the group on account of one other toddler. Soon enough, your child will meet -- and have to deal with -- a wide range of personalities at school.
At any toddler gathering, a parent should guard against becoming an intruding presence rather than a comforting chaperone. Let the children play -- or not play -- together as they wish. Playing alone is as developmentally appropriate as interacting with another child is. If you want to encourage interaction subtly, provide toys that involve group effort: toy food and dishes for pretend meals, a large set of blocks, a plastic building set with interlocking pieces. Or, if you have a solo toy such as a small ride-on car, suggest that one child push another.