How would I create a ‘group dashboard’ or account where several people can update/support an add-on instead of it just being tied to a single user/email address?
AMO has a feature that’s very similar to what you’re asking about: it’s possible to add other AMO accounts to an extension as a “Developer” or “Owner”. After you invite another account the account holder accepts the invitation, they will see the add-on listed on Developer Hub.
You can find more information about this feature in the Add-on Ownership and Signing and distributing your add-on, just be aware that both articles focus on transfering ownership rather than sharing publishing rights.