Besides the obvious talk to her then you cna also accpet their invites anyway. Tbh either she is unawate (unlikely) or she doesnt regard you as part of the group or she isnt as good a friend as you think and just uses you when she hasnt got other people.
On your chat keep your calm and just say you find it odd she doesnt invite you (do not ask to be invited, although you can say they invite you, but you find it odd she does not), at which point she will lie to you and say shes never really thought about it before and that you can invite yourself, she might offer to change her ways and then she will ignore you as usual. Thats what I thought anyway. Places £5 bet.