If the authentication provider you use supports it, you can configure Connect Daily to automatically add users. Authentication providers that support auto add include LDAP/AD, Cookie Signon, and container authentication.
Once you've got your authentication provider working, create a user account that will be a template. This user's settings will be copied to each new user that is created. Set the user level preferences, group memberships, and individual permissions to be what you want.
Now, go to System | Configuration | Security. In the area for Template User for auto-adding users, enter the name of the template user you created. If you're using multiple AD servers, you can specify a different template user for each server by editing ldap.properties.
Once you've done these steps if a user authenticates but does not exist in the Connect Daily user database, they will automatically be added.
If you are using LDAP/AD or Cookie Signon and have set up LDAP Equivalent groups, then after the account is created, the user's group memberships will be adjusted based on their LDAP/AD group memberships. Refer to the Add/Edit Group page for more information.