For an invite-only website I'm hosting, I'm currently looking for a good word for the person who vouches for the new user. The inviting user is really taking some responsibility for the new user, i.e. that they are a good match for the community etc. If the new one behaves badly, both can be kicked out again.
Think about an exclusive club where not everyone can or should be invited, but only people who are a good match. Somebody who invites somebody new, and guarantees that the match is a good one, is the person for who I'm looking a word for.
English is not my native language, but I'm pretty sure that "voucher" is not what one should use here.
So far we came up with "sponsor" or "bailer" as being the best words - or using the term "vouching user".
What would be best in such a case?