In short, if someone sent a guild invite request, and changed their Family name, afterwards (Unsure about character name, too) it won't let you send the contract.

Instead, it will alert with: "Character does not exist" 

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The invites should be ID bound instead of name bound, and if you hit refresh, it should update the names accordingly

Currently it doesn't, and requires you to deny the invite, and have them reinvite. (If you can whisper the same person - If you know their new name)