@alexis or others Periodically I think this question can effect users decision on setting up there structure, especially with consultants. Here are some of my understandings that definitely can be wrong.
Free Version: All participants are counted against the use of member seats whether having the same organizational email or a personal email.
Premium Version: This one seems to be a little more complex but maybe not. Are member seats only taken by users having the same organizational email address or can Team Member/Project members having an email address that is not part of the organizational email address take up a seat in the Premium pricing.