GreenWall allows multiple guilds to share a common guild chat and officer chat. It also replicates achievement spam between guilds. It is designed to be a turn-key system that requires your guild members to do nothing more than install the add-on to participate.
A quick-start guide that shows how to set up the guild-wide configuration for use with a minimum of reading and effort can be found at the following location.
The full documentation can be found on GitHub.