Dd's class is only one third girls. DD plays with most but not all. The three or four she doesn't play with regularly she still gets on with. It's a nice little bunch. I usually do her birthday party at ours and invite all the girls. There are sometimes a couple of no shows but, having MS, my energy levels don't really suit home parties - takes me a week or so to recover. I decided this year to go for an activity venue. Trouble is I can't bring myself to exclude any of the girls. A couple of them have ubiquitous "We'll turn up or not as we like but won't let the party host know" Mums - including a good pal of Dd's. There's also a newbie factor. The cohort of girls is so small the uninvited would know they are The Uninvited. The price of the venue is reasonable but it starts to pinch once we go towards 10. I plan to be a lot firmer by text with an "If I don't hear from you by such and such a date, there won't be a place for your daughter" as the cut off date draws near. Still, what would you do?
What are the charging policies like at the venue? Round here, you book a minimum number of places but can add more on the day. So I usually invite 12 but book 10 type of thing. If this is the case with you, you ouldinvite all but book slightly less places. Or alternatively say on the invites "Please rsvp by the XXX (2 days before you actually need to know) or we will assume you are not coming". And mean it.
A third way would be to invite some boys (if your dd plays with any) - then its a mixed part and OK to leave a few girls out.
If you can afford to invite all, do so, but be very clear about needing responses. Get contact numbers to chase up and remind everybody about the date.
If all the girls accepting would make it too expensive, go small. Tell dd she can invite 5 girls and stick to it. That way the un-invited are the majority. This regularly happens in dd's class and they all seem to be quite ok about it. There are 13 girls in her class and some have never been to each others parties. DD has always invited both boys and girls to her parties, which, apart from her 10th, have all been small.
Thanks for all your comments. DD is very much a No Boys Allowed which is a pity because I am quite pally with some Mums of the boys. I will be firm with cut off date (and mentally prepare for at least one awkward moment on the day when someone turns up ).