Forming an AllianceTo create an Alliance, you would login to your Site Admin and go to the Alliances tab, then click "Add New Alliance"; a popup will be shown asking you to type the url to the site you wish to ally with, auto complete will help you to find the site you are looking for:
One you have selected the site, a Pending alliance will be listed in your Site Admin, awaiting the approval of the other site. Once the other site approves the Alliance it will become active.
Features of an AllianceWith an active alliance, members from your site can login to theirs, and members from their site can login to yours. Alliance members belong to a new account type, "Alliance", which you can configure in the Permissions tab, and belong to a new view level, "Alliance", you can use to control what forum/poll content they can see.
In addition, Events gain a new feature allowing you to choose which events should show up on Allied calendars. Checking this option for an event will make the event show up on all allied calendars, allowing you to easily coordinate for important events.