write xmlrpc function updateGroups that is to be contributed to the upstream. Please refer to the following url for details on Bugzilla::RPC 2.18 functions mapped to upstream Bugzilla::WebService and for 2.18 xmlrpc functions sorted by percentage of calls: https://engineering.redhat.com/trac/bugzilla-3.0-rh/wiki/XmlrpcCrossComparison#a2.18xmlrpcfunctionssortedbypercentageofcalls
This function is not going to be ported to compat_xmlrpc ,, however its functionality can now be accomplished by the 3.2 WebService function Bug.update which we can use to add/remove groups to/from a bug.