Okay, here's how it works. A restaurant has a certain number of chairs and tables, when you make a reservation they save that number of chairs and tables for you. A busy restaurant may not be able to seat 2 people without a reservation on a busy Friday or Saturday, let alone anyone and everyone. Chain restaurants will usually be better able to accomodate you, they tend to be bigger anyway.
Also, if you say you're going to have 15 people in your party when you make your reservation they save 15 seats, so if you then show up with 20 people there may not be enough seats to go around. If you say it's 15 and you come in with 10, then that's 5 seats that have been held all night when your server could have had other customers there and made more money. They may not appreciate that.
Also, if everyone comes in helter-skelter ordering random things at random times you are making everyone's life very difficult. If at all possible meet at the bar and don't go to your table until everyone has arrived. Make sure that the waiter or hostess knows if there are any changes in the number of people or the time, many restaurants won't hold a table longer than 20 minutes if you don't call and let them know you're running late.
Plus, when you make the reservation you can let them know that it's your birthday, lots of restaurants will do something special for you.