Assigning users to a queue can be done on the Group level as well as on the User level. 

1. Group level.

To assign on Group level

Go to Admin => Groups

Click on List to view all the Groups All the Group names will be displayed, click on the desired Group. It will redirect you to the Group details form page.  In the Users field select the users that you would like to assign to the group with the help of the drop-down menu,  Permission can be granted for the users to use selective features or all the features in their TxHelpdesk application by just checking the desired box.. After setting permission, please click on Save  Below screenshot for your reference

2. User level  

To assign on User level

Go to Admin => Users

Click on List to see all the users.  All the user names will be displayed, click on the desired User. It will redirect you to the User details form page,   In the Groups field, select the group that you like the user to be part of. . In the User Permissions field view list of permissions to be granted to the user with the help of a drop-down menu.  Below Screenshot for your reference.  Select the group that you would like the user to be part of. . Below Screenshot for your reference.