Got an issue as of this wipe where players that are VIP are added to the usergroup but their colour remains default blue, yet players that were VIP from before wipe still have their gold names! 

 - Confirmed that players are added to the usegroup correctly.
 - Have been able to replicate the issue with numerous different players (not 1 new player added has had colour changed).
 - Have created an entirely new usergroup with a different colour to test the functionality, still stuck on Blue.
 - Confirmed that it is the latest version and have updated the plugin as of this morning.

Any help would be hugely appreciated!!

Hmmmm... I'm just setting this up for a friend's server, and feel like having the same issue. On a local installation I've put myself into a new admingroup, which is set to prio 0, default is set to prio 1.
The admins should have a differently colored name (just like the vanilla chat of rust does) but it seems to have no effect :-/

Also I can't find where the group-members for the BetterChat-groups are stored... I don't see any mapping file, but as I only use the group "admin" it may just take it from the oxidedata (as it's said to be like this with defaul, admin and moderator)

Yeah, never managed to fix this issue unfortunately :/

Quick follow-up:

I logged into the server and used the command /chat user add <steamid> <desired group> and at this time the players name changed.
The aforementioned player had already been added to the 'usergroups' via RCON automatically, but required me to log on and do the '/chat user add...' command.

This did not used to be the case. Could I get any help with this? Will be totally impossible to manage all the VIP's manually!

Turns out it is case sensitive. I had capital letter both in the config file and in the command, which caused it to fail. 
Had to remove the capital letter from the commands and update config to reflect no capital to fix the issue.