You can add colleagues to an event as either Workers or Managers.

Event Managers can edit the event’s information, add/remove other workers/managers, add/remove guests and delete the event
Event Workers can see the event’s information, record timesheets and log guest attendance

A colleague must have any one of the following roles in order to be added to an event as a Worker:

Steps

Add workers to a new event - i.e. while creating an event

To create an event, you must have any one of the following roles:

Permission to create events (this is granted by the Project Manager or Organisation Admin on the Project Preferences page)
Project Manager of the project that the event belongs to
Organisation Admin

Select My Apps in the top menu
Select Events
You arrive on the Manage Events page
Press the Actions button next to the event you want to add workers to
A dropdown menu appears
Select Edit
You arrive on the Edit Event page
Scroll down to the Event Workers field
Enter the names of the people you want to add to the event as Workers and select their names from the list below
Scroll to the bottom and press Save
You arrive on the Event page where those people will now appear on the event page in the Event Workers box

To add workers to an existing event, you must have any one of the following roles:

Select My Apps in the top menu
Select Events
You arrive on the Manage Events page
Press the Actions button next to the event you want to add workers to
A dropdown menu appears
Select Edit
You arrive on the Edit Event page
Scroll down to the Event Workers field
Enter the names of the people you want to add to the event as Workers and select their names from the list below
Scroll to the bottom and press Save
You arrive on the Event page where those people will now appear on the event page in the Event Workers box