Group memberships are memberships where more than one person is part of the same membership. You can think of it as one lead member + additional anchored members. Group memberships allow you to offer:
Family memberships
Household memberships
Spouse memberships
Partner membership
Parent and child memberships
Other combinations of people
When creating a membership type, you can select to toggle on Group Membership. That allows you to select:
Max. number of additional adults
Max. number of additional children
Define prices for the full group
When players purchase a group membership, they then get the ability to add additional adults and children based on the settings you have defined.