...would you expect to have to pay for their share of the bill?

So a girlfriend of mine always invites a big group of girls out to celebrate her own birthday. She'll order wine and drinks and apps and dessert. But when the bill comes, she never offers to pay it. The first time it happened, we were kinda surprised cause we thought she was gonna pay for the whole thing! She seemed so "in charge" of the menu that night that it seemed like she was treating us. But when the bill came, she didn't even offer. It's been like this every year for 5 years and some of the girls just don't want to go out with her anymore.

Is it wrong of her to expect people to pay her share of the bill? Or as we supposed to pay it, since it's her birthday?