You can use the status Approved To Participate for this purpose. It can be given by the organizers to individual users.
First, please go to:
Overview => Settings => Settings for User Registration => Main Settings for User Registration and Management
... and enable the setting "Approved To Participate" (see image 1).
Second, make sure that the “Registration for Conference Participation” is still enabled, but has expired. Please go to
Overview => Settings => Conference Phases and Deadlines
and check if the setting of Enable Module and Select Start and End Date regarding "Registration for Conference Participation" is enabled and that the date Phase ends on is in the past (image 2).
Now you can provide the status "Approved to Participate" to selected users if you use edit their details (image 3). Every user with this status can now register for participation.
If you also want to disable registration for participation also for these users, please set the phase "Registration for Conference Participation" to Disabled.