As long as 1 user per guild is online, with Guild2Guild set up and on, then all members of both guilds will be able to share Guild Chat. However it is advised to have many guild members install it. The more users the more stable Guild2Guild is. Members without Guild2Guild installed will still see shared chat but will see the name of the g2g relay toon as well as the name of whoever is speaking, ie: "[Jaxom][Fred]: This is normal!" For example, guild chat from guild A will appear in guild B's guild chat, and vice-versa.