First thing to note is that the Membership Plan you are selling will need to be set up as a Group Membership to sell it as such. If it is not, you will not be able to add in an additional member.
There are several ways to add a Member to a Group Membership. We will be going from adding one at Sign Up, and later when one Member already exists in the system and is adding an additional member to their membership.
NOTE: You will only be allowed to add as many members as the plan allows. If you cannot add any more members to a plan, then you either need to go back and select a different plan or the current one needs to be reconfigured to allow multiple members. Plans can be adjusted in the Owner Entity (OE) Tier
There are 3 ways to add in a person to a Group Membership with a member that already has a membership.
When converting a Prospect to a Member.
In the Members Record if the member has a membership that allows additional members.
When Re-signing a Terminated Member.
In each of these options you will need to select the person you are signing up as a new member and the member that they will be joining.