There aren't really any official "local GUE groups" as such. There are a variety of local groups which are loosely affiliated with GUE but aren't really accountable to GUE HQ. Some of them are registered non-profit corporations, others are just an informal email list or Facebook group. The local groups are free to set their own rules about equipment configurations and other diving protocols. But generally divers wouldn't join one of those groups in the first place unless they had already bought into the GUE approach. Part of that approach involves using standardized gear configurations so that everyone can function smoothly as part of an integrated team.Local GUE groups organize dives for their members and part of that is the members using GUE configuration.
That's my perspective looking in.
For the dives we do in Northern California with BAUE there's no point in using sidemount. We have no cave diving or wreck penetration. So everyone uses standard backmount configuration for club dives (boat charters, projects, social beach dives, etc.). Circumstances differ in other groups.