Okay. A guest user list can ONLY be imported, if it was first exported from an Aruba Controller. a CSV cannot be imported.
What you have to do is to use excel to put the usernames and passwords in two columns. In excel, you should also put the commands to add each user in colums around the usernames and passwords. Copy and paste the commands into the commandline on the controller. For example, the format for adding a user (or guest) on the commandline is:
local-userdb add username <username> password <password> role <role>
So what you have to do in excel is create a column that has "local-userdb <space>" in one column, your usernames in the second column, "password" in the third column, the password for that user in the fourth column. Make a list like this : http://community.arubanetworks.com/aruba/attachments/aruba/67/17/1/local-userdb.jpg
Select all the rows and columns and paste into the commandline in the controller.