Sharepoint Online External Group

Hi,

We currently have a Sharepoint 2013 foundation server that we run our small intranet on. We are a small group of companies (3) and each company has their own AD and Tenet on Azure AD. There are about 200 users in total. I have been trialling the Sharepoint Migration tool to migrate over the current intranet which seems a bit buggy but I think it will be enough with a bit of work to do a like for like. It seems pretty straight forward to add in my current users in the org that the Sharepoint site is hosted in. however, I also need to add in all of the other users from the 2 external organisations so they can see the site. Is there a simpler way to do this other than have to add in 130 users manually? Ideally, I was hoping to be able to just apply a group that was created in each external org to the sharepoint site but that doesnt seem possible. Previously (or currently, I should say) as we have forest trusts in place on premises, its pretty easy to do this but now we are moving to the cloud it doesnt seem like it is possible?

thanks,

Paul