I don't know if you would be able to get in without paying first. It's a buffet style service; don't think they are going to allow you to go in just to socialize. It doesn't work like that where I live. But, you can give it a try and see what happens.
If the person hosting the party doesn't cover the cost of your meal and you don't have the money to pay for it yourself then the only thing you can do is go in and socialize with them and wish the person a Happy Birthday.
Thanks! Your answer is awaiting moderation.