Manually assign users to roles in D365

Go to Navigation pane > Modules > System administration (Scroll the the way to the bottom) > Security > Assign users to roles.    

In the tree, select a role, and in the Users assigned to role menu, select Manually assign/exclude users.   In the Assign users to or exclude users from role, users that have not been assigned the role are listed with the Assignment mode set to None. Select one or more users that should be assigned the role.  On the Action pane, select Assign to role. The Assignment mode is updated to Manual and the users now have a new role assigned.