How do joint birthday parties work re: inviting other kids not from the class?

[Nic165]

Sorry if this is the wrong board to post on. I'm thinking of asking another mum if her child would like to have a joint birthday party with mine - they are both turning 5 and inviting the whole class. I figured it would cut down on the cost (split the cost of venue and food) and better for parents (one less weekend going to a single birthday party). But say one of us wants to invite 5 other kids who aren't in the class. I wouldnt expect the other person to include those when splitting the cost of food. Does that person pay separately for any additional kids not from the class? Sorry this is the first time I'm considering a joint party and probably over thinking it.

It would really depend, IMO, on whether you were hiring a hall and catering it yourself or having a 'pay per head' party, such as a soft play party.
In the former I'd not worry about the catering being head counted- at 5 they don't eat that much at parties and the difference would be negligible but at a 'pay per head' party each pay for any extras out of the main class list and share the cost of the class invitees.