For the Leader
Generally, there are 3 functions that are available in guild management interface.

Recruit new members
Guild leader has the right to recruit members by approving applicants application. He/She may also reject the applications.

Remove existing members
Guild leader can kick out any members who broke the guild rules.

Disband Guild
If a guild leader wants to disband the guild, he/she must enter the security password that was set in the beginning.

Tips: By pressing Alt + C, you will bring up the guild menu which allows you to recruit new members, kick existing members and to change the motto of the guild. You also have an option to disband the guild, wherein you will need to enter the security code you entered while creating the guild.

For Ordinary Member

Leave a Guild
To leave a guild, press Alt+C and click on "Exit". Confirm your decision in the message box that pops up.

Miscellaneous
To engage in conversations with your guild members, you may either use the guild channel, or press Alt+F to bring up the friend list and double click the name in the drop down list.

To make a multi-member conversation among some members, follow the above step of using the friend list and then drag and drop names into the message box to add members to the conversation.