Memberships can be used for all types of things, and may or may not require payment by the member. Associate memberships are memberships underneath another membership, generally used for family. If memberships are enabled in your copy of VSys, click on the Add membership link on the Memberships panel, or on the Memberships link on the left navigation bar to add a new membership to the current person. Right-click on an existing membership to edit or delete it.
If the Memberships link does not show as an option on the left navigation bar, check the System preferences and feature enabling under Feature enabling and make sure Memberships is checked. Also make sure that at least one Membership Type is defined.