18.104.22.168 GroupMembership Table
TABLE GroupMembership( SiteId uniqueidentifier NOT NULL, GroupId int NOT NULL, MemberId int NOT NULL );
SiteId: The site collection identifier (section 22.214.171.124) of the site collection containing the principals and site groups.
GroupId: The group identifier of the site group that the principal specified by MemberId is a member of.
MemberId: The user identifier (section 126.96.36.199) of a principal that is a member of the site group specified by GroupId.