By default, all extensions are in the same call group. (the undefined call group)
Add a "call group" name to the extensions you want to be able to pickup calls for.
In the above screen shot, any extensions part of "group1" can pick up ringing calls for any other extension in the same group.
Extensions can be in multiple groups by separating the group name with a comma. ie: group1, sales, group4, ninjas
call groups names can be any logical name you chose.