Guild communication and management can be done through the guild commands. Players with adequate permissions can get starting information about the guild, send guild chat messages when they're members, invite others to the guild, promote/demote them, remove them, set a message of the day, change the guild leader and even quit the guild.

Type /ghelp for a list of guild commands. Note that some commands can only be used by the guild leader.

/ginfo - Gives basic information about your guild
/g - Sends a chat message to all members of your guild
/o - Sends a chat message to all officers of your guild
/ginvite - invites another player to join your guild
/gremove - removes a player from your guild
/gpromote - promotes a player one rank within your guild
/gdemote - demotes a player one rank within your guild
/gmotd - sets the guild's message of the day
/gquit - removes you from your guild (how to quit a guild)
/groster - gives an entire guild roster
/gleader - sets another player as the guild master (guild master only)
/gdisband - disbands your guild (guild leader only)

Related posts:

1. General command-line commands for World of Warcraft
2. Chat communciations on World of Warcraft
3. Maximum guild size in World of Warcraft