Sorry this is long, I'm no good at being brief.
I have twins in reception. They're in separate classes and have separate friends but the classes do intermingle and they have crossover friends from nursery so there is a fair bit of playing together. One of the twins is very outgoing and popular, gets on with everyone. The other is more reserved and less socially skilled. When the reserved-twin's friends come over for a play date the outgoing-twin tends to take over and reserved-twin gets left out a bit.
Reserved-twin does have a tendancy to feel excluded at the slightest hint of being ignored whether it was deliberate or just because everyone was in the midst of something else - which leads to withdrawing from the game and tears. Outgoing-twin just doesn't notice these sorts of slights (real or imagined), is more flexible and will happily say and hear "no" without feeling it's some kind of end of friendship, so stays in games and just joins in with whatever is being played.
When playing one on one with a friend reserved-twin is much better.
I'm concerned the group friend situations sometimes become bullying against reserved-twin (at least from reserved-twin's perspective, not sure I think 4 year olds can be thought of as being bullies as implies a degree of malice I don't ascribe to them). It's worse with some friends than others. Reserved-twin's best friend being one of the most problematic, but will also happen a lot with new friends like the instant friends they make at the park.
I have tried to arrange separate playdates for them but logistically it's very difficult and would mean about one a term! I'm starting to supervise playdates a little more closely and pull up any deliberate exclusion etc. to nip it in the bud, but reserved-twin will still take umbrage at normal interactions too.
Are there any strategies to help reserved-twin develop a bit more resilience and say "no" appropriately without taking it to heart so much? Is this what I need to be bolstering?