A Booking has two assignee roles:
Sales - the team member who sells the Booking. This is the field previously called Assignee.
Operations - the team member who services the Booking once it is confirmed.
Both can be set at the same time, so the Booking no longer has to be reassigned when it moves from sales to operations. Your existing Assignee values carry over to the Sales role, and Operations stays empty until you set it.
For full detail on how the two roles behave across workflows, the Event Diary, and reports, see Sales and Operations Assignees on Bookings.
If you need more than two
It is not currently possible to add a third assignee. You can use Custom Fields as a workaround to tag additional users on a Booking. They will not receive notification emails the way an Assignee does, but you can add these filters to Views in Bookings, allowing you to create a Custom View so people can easily see if they are also involved in the Booking.
It is recommended for each User in the Picklist to Save a View that includes the filter to show if they are tagged on a Booking.