@The Pentacle Queen,
Mixing friends can, indeed, be fraught for many reasons.
One reason is they tend, in my experience, to size each other up, a bit like dogs meeting, but with less bums on show. Especially the closest to you in each group.
It can go brilliantly...it can go badly.
Just remember, they will decide how much fun they have.
The worst I ever had was a Maoist and a Trot together. They fought so badly that the Trot, who was pregnant, accosted me in the kitchen, panicking that she had been aggravated into a mis-carriage.
A plague on both their houses!!!
Other times it's been tollicking good fun.
Good luck and have fun...and Happy Birthday!!!