It’s an invitation to be invited to attend a group event!
If you received a personal invite yourself that is… It’s kind of tough when you’ve heard about a party or get together and then realize that’s it’s by invitations only. You could always politely ask a friend who’s going if you can be their guest or tag-along if it’s acceptable. Or you could just ask the host if there is any room at the event for you to be able to attend as well. Most of the time it just takes a little skillful finesse and refinement to your approach and then you’re in the party! Good luck!!!