The following C# code iterates all groups in your domain and removes each member from them:
AppsService service = new AppsService(domain, username, password);
AppsExtendedFeed groups = service.Groups.RetrieveAllGroups();
for (int i = 0; i < groups.Entries.Count; i++) {
AppsExtendedEntry group = (AppsExtendedEntry)groups.Entries[i];
string groupId = group.getPropertyValueByName("groupId");
AppsExtendedFeed members = service.Groups.RetrieveAllMembers(groupId);
for (int j = 0; j < members.Entries.Count; j++) {
AppsExtendedEntry member = (AppsExtendedEntry)members.Entries[j];
string memberId = member.getPropertyValueByName("memberId");
service.Groups.RemoveMemberFromGroup(memberId, groupId);
}
}
Claudio