Great Idea so +1 for sure.
I would like to include 2 types of groups, open and closed. This would work in the same manner as open or closed towns.
This way when the person who opened the chat logs off it doesn't close but the reassigns itself to say the player who has been longest on the group chat. By being open other people can just join.
On the colours, just assign a colour to the "GC" like "L" for local is and "G" for global.
Another nice feature would be an auto invite for players on your friends list or from the same town.
This suggestion by The_Unknown has the potential to help improve networks for friends, towns and the general community. All of which will improve the gaming experience and so result in player retention and activity.