Hi Marcel,

This can be accomplished by making sure the agent is set to actively receive tickets in their groups. If your account has the Manager role, you can quickly make sure that all agents are active in all groups they're in. Click on the Dashboard icon and choose Group Dashboard or Employee Dashboard. Depending on the dashboard you choose, you'll either get a list of employees or groups. You can double-click on the employee or group and use the Select All checkbox to make all groups or agents actively receive tickets. 

An agent can be set to Pull from Queue for the group, and as long as they're active in that group, any tickets they take from the Queue will remain with them through customer replies. 

If you want agents to only keep SOME tickets, they can remain inactive in the group and use the Pin Ticket option to keep the certain tickets. 

You can learn more about Active Groups here: 

Active Groups Versus Inactive Groups for Agents 

I hope this helps!