I just upgraded to the Business Class, paying the annual fee of $120. The billing says that I am able to add "guests" who will not have to pay the annual fee as long as their participation is limited to one board.
It is, however, not clear how to add a guest to the board. The choices under "invite" include adding "members" (which cost an additional subscription) and "observers" who do not have the ability to contribute on the board.
Please help me understand how to add a guest who can have the ability to contribute the board without having to pay the annual subscription for a new member.