A person that does one or both of these 2 things:
1) turns up at an event mainly a party without getting invited by the host(s).
2) Asks the host if they can come to an event,even though it's invite only and they weren't invited in the first place. *
* this puts the host in an awkward situation where they have to say 'yes'.
Person 1 : hey isn't that Toby over there. Did you invite him?
Host: yeah it is. I didn't ... Ffs
Self inviter: hey Sarah , heard about your party is it ok if I turn up?
Host: urm.... Sure I guess.