In the "User Account" module, what happens if "allow signups" is not checked?
Using Jamroom
Users can only signup to quotas that have the 'Allow Signups' option checked (if more that one, they get a select option). If there are more 'non-signup' quotas that users can request to join you'll need to create a custom page that lists them. Maybe you could use the CustomForm module to do this so as to have the application form built in.
hth