If you want particular Event Roles to be available on certain events, add those Event Roles to the Event Categories of those events. When you add Event Roles to an Event Category you choose whether that Event Role is available for Event Guests and/or Event Staff (i.e. Event Managers and Event Workers).
Steps
First, create the Event Roles you want to add to the Event Category. You can see which Event Roles already exist by going to the Manage Event Roles page.
Once the Event Roles you need exist, go to the Edit Event Category page by:
Selecting My Apps in the top menu
Selecting Event Categories
Press the Edit button next to the Event Category that you want to add the Event Roles to
You arrive on the Edit Event Category page
Scroll down to Section 4: Event Attendance
Tick the Enable event roles tick box.
Once ticked, a box will appear which lists the Event Roles that exist within your organisation
Use the tick boxes to decide whether each Event Role is available to the Guests and/or Event Workers & Managers of the events created with this Event Category. (You can make an Event Role available to both the Guests and the Event Workers & Event Managers)
Press Save
When you go to the Event page, you see that there is now the option to assign an Event Role to Event Guests, Event Workers and Event Managers according to the options you chose on the Edit Event Category page.