The best way to teach your daughter, and any other child in your care, to get along with others is to keep her with the other children. When she is sent to her room, she gets to play alone and doesn't have to share her toys, etc. In a sense, it's like being rewarded for her aggression since the thing she is having a hard time with is being with peers.
Interacting and playing with others is hard for most toddlers (only children or not) and she's still at the beginning of this learning process. When I worked with toddlers, I spent a lot of time modeling appropriate behavior, helping them find words, and mirroring feelings to help them learn to understand their feelings. For example, a child picks up one of your daughter's favorite toys. She yells, "NO, MINE!" and has her hand up in the air about to hit the other child. If you're right there, you can catch her hand and say something like, "I know you are upset and hitting is not OK." (To Ella) and to the other child "Ryley, that is Ella's favorite toy and it wasn't supposed to be out of her room. Ella, can you ask Ryley to please give you that toy and get something else for her to play with?" Here you are demonstrating pro-social behavior by keeping her in the situation to work out a solution. It takes some time and she'll get it. She will probably start to warm to the other children faster because she has more opportunities to interact and she'll begin to learn that everyone's feelings are important (a hard lesson for toddlers and some adults unfortunately).
Just keep at it. She is still very young and this is a great time to introduce other children to her environment and help her figure out how to handle that. Only child or not, this is an issue for most toddlers. It will take time and with your loving support, she (and the other kids) will learn valuable life lessons. Good luck.