Log into switch and go to Ring Groups. There will be a drop down window by Ring Groups, towards the left hand bottom of the page you will see another box that has Members in it. To the right side of the Members you will see two drop down windows. The second drop down is the one you want to select to find the extension you want to add to the list of Members. Once you have selected the extension then you will need to hit Add. Once change is made hit apply
Save and make sure to notate the ticket number and what you did.
For in-depth over view of extension