There are two ways that your shop can be added to the Alliance.
1) You'll be invited. The invitation will most likely be sent to your shop mule AND you, so that way, you can use your mule to keep your shop summary in the Shop Corner (so your staff can also update it), and then you yourself can participate in the rest of the Alliance's forums.

2) You can be nominated or request to join. Nominations must come from someone besides the shop's owner or staff.
Once you are nominated or have requested to join, your shop will be reviewed by Jaina and the staff, and if accepted, you will be invited or accepted, respectively.

Also, once you have joined the Alliance, you will be asked to place an affiliate banner for the Alliance in your shop, preferably somewhere on the front page (especially since that's where most affiliate lists are anyways). This way, you can spread the word about the Alliance and help others find new shops, including your own.