a paid-up member, etc.
paid-up member (of something)
a (fully) ˌpaid-up ˈmember, etc.
1 a person who has paid the money necessary to become a member of a group, etc: The society has got over 10 000 paid-up members.
2 (informal) a strong and enthusiastic supporter of a group, etc: He is a fully paid-up supporter of the Green Party.