You can assign agents to specific queues from Agents, Teams and Queues under Configuration.
To assign an agent to specific queues, click the gear icon of the right side of the agent to be assigned, then select Assign Queues.
You will then be shown all your queues. To assign the agent to a queue, click on the tick box next to the queue name, as shown below. You can set the priority level of the queue from 0 to 100 (lower/higher) compared to other queues. You can also tick the box Select All to assign all queues to the agent.
Click Save Changes once you have finished editing queues for the agent.