[ENH] PluginGroup: add parameters for when membership in a group is pending (e.g., when payment to join a group through PluginMemberpayment is still outstanding) or not.

git-svn-id: file:///var/local/code.tiki.org/tikisvn/trunk@49814 b456876b-0849-0410-b77d-98878d47e9d5