In the Control Panel, admins with Settings and Roster admin rights see a link to the "Idle Monitor" in their "Member Tasks" list when they click on "Site Members."
Using this tool, admins can configure warning e-mails that are sent out after a specified number of days of a user not logging in to the site. Typically these e-mails warn the user that they have been inactive on the site and face auto-disbanding or other penalties if they don't participate more freqently.
Optionally, the tool can be configured to automatically disband a user after a number of days of inactivity, also sending an e-mail. Typically the e-mail will contain instructions on how to re-apply to the site, as well as an explanation of the guild's activity policies.
Exemptions can be configured to ignore specific users during idle processing, protecting members who have gone AFK for extended periods with an explanation that is acceptable to the guild leadership. Public users who have not been approved as at least Associates, and users who have access levels of Officer or above, will never be processed by the Idle Monitor, so it is not necessary to add them to the exemptions list.
The actual process that sends out warning e-mails and (optionally) auto-disbands is managed by GuildPortal and runs every other night.