A group plan is sold as a bulk membership package to a group plan sponsor.
For example, a company would like to have 100 of their employees to have a membership to your site. The company would pay you upfront an agreed upon amount to let their members join. The group plan would be set up in MemberGate with an email qualifier (anyone joining would have to have firstname.lastname@example.org), the maximum number of members, and an expiration date (all group members would expire on the same date).
Anyone who wanted to join through the group plan would not have to pay, since the company already made the payment for their employees.
To setup a group membership plan, follow the instructions in this tutorial: //www.membergate.com/members/174.cfm