This is a simple process. Here goes, step by step...

1. Navigate to the main Fluid web site. Clicking the welcome tab above is a good way to get there.
2. Go to the Zope Management Interface (ZMI) for the Fluid web site. You do this by editing the URL showing in your browser's navigation bar so that instead of ending in ".../Fluid" it ends with ".../Fluid/manage". Yes, just add "/manage" to the end. You will see a dual-pane interface with navigation in the left pane, and lots of check-boxes and stuff in the right-hand pane.
3. Click on acl_users in the left-hand pane. This takes you to the user management page for the site.
4. You should see a list of the currently defined users (with check-boxes). There will be two buttons at the bottom of the list: Add... and Delete
5. Click the Add... button. This takes you to the Add User page. Type in the username for the new user along with a password (twice). Ignore the Domains entry. Highlight "Manager", "Member", and "Reviewer" in the Roles area. Click Add
6. You've just added a user. Repeat as needed to add more.