Hi Dentvincent:
This is somewhat similar to what happened to both DD1 & DD2 in Y1/ Y2 - there seems to be settling out of the pecking order around this time and sadly girls can be particularly mean.
First I think you are doing the right thing by encouraging your DD to do activities outside of school where she can make friends with other children. This is a brilliant way of sending the message that there is more to life than primary school.
Play dates and birthday parties are bonding activities that most children adore being invited to - so if you can have your DD's freinds around every now and then - at the weekend or during half-term, maybe make a day of it your DD and her friends will really enjoy it.
Now 6 friends is a good number but it can also become a power struggle (as there will probably already be a dominant friend in the group). My advice to you is to remind your DD that she can things in the play ground on her own: point out that there is hopscotch, that she can play on the bars, play on bikes/ scooters, skip rope, read, etc.... If she's at a loose end and hasn't started playing with anyone she can start doing one of those activities. My advice to DD1 who found squabbling friends hard going and her BFF started to be away a lot for training (she's G&T in sport) - so DD1 decided it was better to play on her own for a bit, but would always say 'yes' if someone asked to join her. In that way she got a reputation for being 'the nice one - who always plays nicely with everybody' and now Y5 she never is worried about playing on her own.
I think the most important advice I can give you is to encourage your daughter to be very flexible about her friends and generous about their need to sometimes do something different from her. There are probably 29 other pupils in her class and children from other years she might consider friends. She should join in the big soccer game or help a Class R child on their trike etc.... and make as many friends as she likes.
One thing to gently discuss is does she start to get anxious at recess/ lunch time about who she will be playing with/ eating with. Some children try to organise who will play with them and other children don't always like it and respond by simply not playing with the over bossy child (who may not well realise that's what they're doing). It's really important to teach her that she can't control people and force them to do as she wills.
HTH