You can configure the system so that new users can create their own accounts, but the accounts are not activated until a site admin approves them.
To enable this mode of operation, add the following line to /opt/collabnet/teamforge/sourceforge_home/etc/sourceforge_configuration.properties:
sf.approveNewUserAccounts=true
Once this line has been added to the file, restart TeamForge for it to take effect.
Please note that site admins can still create accounts for new users and they will not be held for approval. Also note that the user will receive an email from TeamForge telling them to confirm their password by clicking on the given link, and the link will not work. The password is properly set on account approval.