To communicate directly with other people you have four commands which effectively cover the range of communication.
SAY <message> - Tells everybody in your location the message.
SAY TO <player> <message> - Tells a specific player in your location the message.
SHOUT <message> - Tells everybody in Avalon your message.
TELL <person> <message> - Tells specific people your message.
WHISPER <person> <message> - Tells specific people in your location.
TELLS - Review the last dozen or so tells to you.
CT - Clears the contents of your tells log.
WHO - To tell you who is on Avalon at present.
HELP CALLING - To learn about the OO calling/communications.
Note that your tells log lasts at most up to twenty four hours, whether or not you clear it yourself. See also HELP WHO for further information on discovering who else is in your vicinity (potential allies, or possible foes) at any given time. Read through HELP MESSAGES to find out about the simplest way of communicating with others (whether or not they are in Avalon when you are) and/or receiving missives from others or Avalon-originated information that may be important to your character; occurrences while you were not logged on.
You can communicate with intelligent CCCs of Avalon (See HELP CCC) using simple emotive commands. They tend to understand NOD, YES, SHAKE, NO, GREET, HUG, SNEER and other such simple emotions. Many give out quests and this is one of the staples of any character's development: see HELP QUESTS for more details on the most popular method of experience gain and wealth accumulation.