On Carnival "bar cards" are available only to groups. And everyone in the group has to purchase the card. Those not in the group are not eligable. Correct.
So... you're "inviting" people to join your group... which really translates to advertising a group cruise, so you can take advantage of the amenities a group qualifies for.
Though I wish you luck finding enough people to form a group, so you can buy the bar cards, soliciting them here isn't allowed. Sorry.