In the Module Settings of the Events module, you'll have a switch 'User Selection Criteria':
Remember that an Event is based on an Event Type and each Event Type will have predefined Groups.
When the switch is turned Off, only Users from those predefined Groups on the Event Type level will be available to be assigned in roles when creating/editing an Event.
Whereas when the switch is turned On, the same switch will appear in the Events Form Page when creating a New Event or when editing an already existing Event:
If you leave the switch Off, only Users that are part of the predefined Groups on Event Type level will be available to be assigned in roles.
When turning the switch On, all Users in Scilife will be available to be assigned in the roles of the Event: