There are two options available for assigning roles and permissions to users.
Option 1: Assign multiple users to a role
In the pop-up window that appears, tick the names of the users you want to assign this role to.
You will also need to assign a Team to those users who are assigned the "Agent" role.
To un-assign users, simply untick the names of the users you wish no longer to have this role
Option 2: Assign a role to an individual user when adding/editing a user.
From here, you can assign a role to a user during the process of adding a user, either individually or via the bulk import users function. You can also assign/un-assign a role to a user by editing their user profile.