Consider the use of capability tokens instead of username/password pairs to grant access to features. This would allow someone to sponsor someone else without sharing any information.

Mojeek generates a signed token that IS authorization, you match it against a list of valid tokens, with no other information required.

This could be used to federate subscriptions, and hosting of front ends such that they could stand alone without having any knowledge of the users, while only serving valid users.

Flickr has something like this that grants access to an otherwise private portfolio, yet the user can revoke it if necessary, without ever sharing passwords, usernames, etc.