any user remains a non-billed seat as long as they are invited to 2 or less workspaces.
should a user have a business having multiple stores, the multiple stores can be setup as subfolders within a single workspace.
a husband and wife, etc, both having personal info where each is allowed to see the other's info can be organized into subfolders within a single workspace.
any user can only access each workspace they are individually invited to, so a business partner (2nd user) that is invited to a (1st) user's workspace for their business could not also access a seperate workspace which is the personal workspace of the (1st) user - without being specifically invited.
the list of invited users of a workspace are the only users that can access that workspace, there is absolutely no crosstalking or merging of access due to mutual invitations ever.

in the case where there are 3 or more workspaces, where a non-billed user would need access to 3 or more of those workspaces, either the invitations could make use of tertiary email accounts for that user (with no email account being invited to more than 2 workspaces) - or the user would have to become a paid seat.

a single workspace can have many invited users with no effect on billing, so long as each invited user is not invited to 3 or more total workspaces.