Thanks for your question. When you have any kind of celebration at a restaurant, the guests should understand in advance that they are responsible for paying for their own meal.
If you have enough money to rent out a space in the restaurant (if they charge for that), and to pay for everyone to eat, that's great. But most hostesses choose to have a party at a restaurant to cut down some of their own expenses.
Just make sure that when you send out the invitations, you let your guests know that they can order something from the menu at their own expense. Depending on how many guests you have, you could provide an appetizer tray for everyone and use the cake as dessert, and inform your guests that they will only have to pay for a main course and a beverage if they choose.