When viewing a ticket, you can assign it to one or more agents. An agent must have access to the mailbox containing this email in order to be assigned.
When viewing a ticket, you can use the controls in the right sidebar to change the assignment.
You can also assign multiple tickets at once. When in the list view, select multiple emails, then choose the "Assign to" dropdown menu above your messages to assign multiple tickets to a single agent in one go.
When viewing a ticket, use your mouse to hover over the agent's name in the assign menu. Click the small "x" to remove the assignment.