Hi Laurie,
Each User will use the number(s) assigned to them as the Outbound Calling Address by default. So the way to set different ones per User is to assign the numbers to the User.
For groups, it is currently possible to set an Outbound Calling Address for a Queue, which will affect all outbound calls placed on behalf of that Queue.
It is also possible to set an Outbound Calling Address for a Dialer campaign.
The Outbound Calling Address can also be provisioned on a Trunk. It can be set to always be used or to only be used when another address was not available. If you are using PureCloud Voice, this would be set on the PureCloud Voice trunk and would apply to all Users.
If you are not using PureCloud Voice, you could configure multiple trunks with different Outbound Calling Addresses and assign them to different Sites. You could then manage the default Outbound Calling Address used for a given User by controlling which Site their phones are assigned to. It's important to note that this would not map the Outbound Calling Address directly to the User. If the User selected a phone in a different site they would inherit the Outbound Calling Address used on that Site's Trunk.
There is currently no functionality to assign an Outbound Calling Address directly to a group of Users.