There are a few group roots configured in Umbraco for use with Membership:
security://profiles
Used to store profile data by the Hive Membership Wrapper. Profile data is additional data for any member/user that cannot be stored in the ASP.Net MembershipProvider
security://user-groups
Used to store the data for back office user groups/roles.
security://member-groups
Used to store the data for members' groups/roles.
security://member-types
Used to store the schema data for member types
security://membership-data
Used by the UmbracoMembershipProvider to store the ASP.Net MembershipProvider information
security://users
Used to query the Hive membership provider wrapper for back office users
security://members
Used to query the Hive membership provider wrapper for Umbraco members