If you wish to ally together with a group of friends, you may form a Alliance. Being in a alliance offers several advantages; however, there are also downsides.
Alliance members can selectively deploy 10% of their troops for the sole purpose of defending other alliance members who also elect to share theirs - in doing so, alliance members can significantly boost their defensive power against attacking dominions. Beware - an enemy engaging in a Ambush will be able to bypass these extra defenses, including the 10% you shared yourself!
Your alliance's leaders can choose to form strategic alliances with other alliances, allowing alliance members to send an unlimited number of aid shipments to their allies as well as send goods to dominions significantly larger than themselves.
However, being in a alliance also opens the possibility of war, allowing members of enemy alliances to attack you mercilessly (and also allowing your alliance to do the same).
When you wish to create a clan, simply enter a short name (no longer than 8 characters) and specify a password which other players must enter in order to join.
If you wish to join a alliance which has already been created and you have been given its password, simply select the alliance from the dropdown list and enter the password in the space provided.
If you have received an invitation to join a clan, simply locate it in the Invitations table and click the corresponding join button.