thanks for posting about this problem. I just made a 3 player game with 1 subscriber and 2 'free' users, and they could participate wihtout problem.
Can you tell me how you crated that table? The player with the subscription should be the one creating it (although it may work if another player does), and the other 2 need to join.
Can you also check that you actually still own Prosperity? The free year for the old accounts has run out. Go to the "Account" tab and see if the Prosperity box is in color and has an end date below it (then you have it), or if it is in grey and has a "-" below it (then you don't have it).