A mailbox associated with a group is called a General Delivery Mailbox.
A group member can log into the GDM and manage its voice message content
A group owner can make changes to the group membership
You need to be a mailbox user before you can login to a GDM, because GDM option (option 9) is available only after you signin to your mailbox.
CCIE Voice notes 
I’ve tried this but option 9 isn’t available to the people in the group that is subscribed to the GDM. Any ideas?
Make sure the users are Members. Only members get messages, not owners. Owners can only add members, not get messages.
This is totally the answer I needed. I had no clue what my user was referring to when she said her manager presses 9 to check voicemail on their main line. I found this owner/member thing yesterday in their CUE system and added her to the group as owner. When she still couldnt check it, I found this blog on the difference between owner and member. Thanks!
They have to be subscribed as Members to be able to check messages and Owner to change Greetings.