You can see the number of collaborators and send reply to multiple users via adding Additional Users in your ticket
Collaborators
To view number of collaborators in the ticket, just look at the Collaborator section on the right side of the ticket.
Users
While opening tickets from email channels, if you mark other users in CC(carbon copy). They will be added to as additional users and they can also send and receive replies on the same ticket.
You can also add additional users by clicking add button of Additional Users
