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