How can I make all the extensions in a group ring at the same time?

You can assign any menu option to ring one or more extensions. Say, for example, that you prompt your caller with "for customer support press 1" and you have three people who can handle customer support. Let's say they are at extensions 12, 13 and 15.

You can use function 44 to assign menu number "1" to ring to 12, 13 and 15 simultaneously. As soon as any of those extensions is answered, the others stop ringing.

If no one answers then the caller will hear the greeting you have in the mailbox of the first extension in that list, extension 12 in this example. Then they can leave a message in that mailbox.