Allow access to SharePoint to all users from a separate tenancy

Hi,

Could someone advise me on the best way to allow external access to SharePoint sites created in Tenancy A from all users Tenancy B?

I can see how to invite individual users, but want to know if there is a way to add all the users from Tenant B into a distribution group created in Tenant A.
As you can't have a domain validated in more than one tenancy I can't see how I can get all the users from Tenancy B into Tenancy A.

Aim: To add users from Tenant B into a distribution group in Tenant A and add that group to the External Users Group created in SharePoint.

Is there a simple way of bulk allowing access to a SharePoint site in one Tenant to users from another?

None of the scenarios in TechNet Topologies for Azure AD Connect appear to help and would need the domain validated in the tenancy.

There may be an easier way that I'm missing?

Any guidance appreciated
Kind regards
Dan