Facebook SEN fun London is worth a look. Also has links to other groups which may be more appropriate for early teens like your girl.
I too have a child diagnosed with autism. He is six but gets along with a very diverse range of ages and abilities and is very social but has no siblings. The most obvious issue he has is speech delay. I would love him to have more playdates. We have toys, a trampoline, an inflatable pool in the garden but what he loves most of all is the company of other children while mums / dads / carers have a cuppa.
Please do get in touch anyone reading and wanting to team up.