Under Configuration => Telephony => Configuration => Extensions: Select the "Pickup Groups" tab

Locate and edit the intended pickup group and add the users extensions to both the "administrator" and "members" and save this. 

Now commit the changes. 

With pickup groups there is a requirement to restart the telephony service which takes 2 seconds but will cut any active calls so its best to do this after hours or you can use the "when convenient" option and this is done as follows:

Under Configuration => Telephony => General: locate the "Telephony Service" and select "restart service"

Restart now: will immediately restart the service and will cut any active calls
Restart when convenient: will wait for 0 calls to restart the service