An Unusual Invitation

Sometimes we get questions in the Entertaining Forum that I consider puzzlers. Joy starts out by saying "it's almost customary to have a birthday dinner where everyone pays for themselves but does not bring a gift." (Actually, it's not that customary in my circles of friends.) Then Joy follows with her question: "My invites are semi-formal. How can I word this to make sure everyone does understand?" This does not sound like a semi-formal event to me, so I'm not sure how she should word these invites. Can you help her? If so, you can post your response in the Forum for How Do I Word This?