Children don't really start to socialise until they are 3 or sometimes 4. While they may play 'together' before this age, the play is what we call paralelle - so they will sit and play beside each other, but not interact with each others play.
There are always exceptions and some children may socialise earlier, some later.
Some children take longer to mix with others, she may be taking alot in while she is standing back watching on her own.
We had one 4 year old boy do exactly what your dd is doing, he took several months of attending to finally form friendships and now plays very confidently with others. He would stand alone alot - as much as we tried to get him to play with others, you can't force it and in the end it will usually happen naturally.