Navigate to the product details page of the specific scheduled activity you wish to assign staff. Pick a date and time slot to narrow down to the specific instance you wish to assign staff. 

Here in the Actions for Time Slot area you will see a Staff Management section. If you do not see this section your user may not have the correct permissions to be able to manage staff.  

From this section select the Add button where you will be presented with two fields. 

First, select the staff member to assign. Next, select one or more roles from your pre-defined list of roles. Each staff member must have at least one role, can have multiple roles, and can share roles with other staff members assigned to the same activity instance. 

Select Save Changes to complete adding this staff member to this activity instance. Repeat for all staff members required.