In the game you will have the basic "say" commands and group chat commands that let you talk to all within a certain radius. Added to that will is the ability to send messages to single players who are currently online and who are members of your Realm. You can also join Battlegroups and Chatgroups which will facilitate communication between players with a common goal.
Due to the competitive nature of the game, we do not allow inter-Realm communication in Camelot. The other realms are your enemies, so there is no need to talk to them. We've learned from our other PvP games that if you allow players to communicate with players from other sides, cheating will certainly follow.