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