Please note that you can also choose directly which user shall be allowed to register for participation.
Go to
Overview => Settings => Settings for User Registration => Main Setup for User Registration and Management
scroll down to "Settings of Specific User Rights" and
and activate the option
"Show user statuses "Approved to submit" and "Approved to participate"Now you can give users the status "
Approved to Participate" if you edit the user details. Users with this status can register even if the participant registration phase is disabled (so this setting is even more selective than the above).
You can now deactivate the phase for registration for participation for all other persons here:
Overview => Settings => Main Settings for Participant Registration
You will find information on how to limit the number of participants here:
http://www.conftool.net/ctforum/index.php/topic,222.0.html