George,
Adding and inviting to a space are two names for the same thing. Inviting non-Podio users is the core case we want to control for now, I'll check if there's a possibility to open up for letting external developers add existing Podio users to work spaces.
This would make sense, since Nick is right, this workaround would work:
1) Authenticate as existing user, invoke
https://developers.podio.com/doc/space-members/request-space-membership-6146231
2) Authenticate as admin of the space, invoke
https://developers.podio.com/doc/space-members/accept-space-membership-request-6146271
The last part is tricky, since you need the space_member_request_id which is currently only available in the notification the space admin gets (see
https://developers.podio.com/doc/notifications). Clearly this is not the way we would recommend you to add existing Podio users to a space, but the point is that it is possible, so we might as well consider making the easier way of doing this accessible for external developers.
Cheers,
Casper