If you would like to synchronize your friends, there are two simple solutions for this.
Do not forget to switch on friends feature in AtomChat Admin Panel -> Settings -> General. Now Search for In Contacts tab, show & from the dropdown, you can select either All Online Friends Or All Friends (Online & Offline). Click 'Update' Button to save the changes.
Comma separated UIDs of each friend
To verify, visit the AtomChat Admin Panel -> Users -> User -> Edit to view the friends.