Bulk upload users
From Joomla! Documentation
How to bulk import users
There are extensions that do this, but if you are just going to do it once or occasionally it's just as easy to do this.
There are three tables that need to be updated when a user is created. jos_users jos_core_acl_aro jos_user_usergroup_map
In jos_users you need a record for each user that looks like this if you are using CSV: "id","name","username","email","password","usertype","block","sendEmail","gid","registerDate","lastvisitDate","activation","params"
You can leave most of these blank but assuming you are creating registered users you need to set usertype to "registered" and "gid" to 18 and, of course, name, username, email and password you will want to fill in, especially if you are in a situation in which you need to tell people their initial passwords. You will probably want to set activation to 1 for all users unless you plan to go through by hand and activate individuals. Since all Joomla 1.5 installations have a user with "value" of 42, I would recommend starting id with a value of 43, assuming that you don't have any existing users.
Jos_core_acl_aro looks like this:
Here you need to have a record for each user where "value" is the same as the id in the jos_users table. The other ones you can leave blank.
In jos_user_usergroup_map you list each user and what group number they are in.
Typically this would be
"user_id","18" for all of your records
18 is the group for registered users. 19 is for authors 20 is for editors. You can see the rest of the group types in jos_core_acl_aro_groups. If you want to use a group besides registered you will need to adjust the gid in jos_users to the appropriate value and change the usertype to "special."
If you have a csv, sql or other compatible list of your users with these three tables properly set up you can easily import them using PHPMyAdmin.