Hi,
Currently the ways I have tried:
1. Use the "invite" button on the instance's user management where you have to manually spell out all the email addresses without any confirmation, that this is one of a managed users. Spelling errors could be costly.
2. Navigating to a Jira group that grants access and try to add managed users there, but it only searches for users, that already have access to the instance in some form, but will not search through all managed users (claimed via domain).
3. Even going to the managed users list and clicking on the user in the list is not an option, as it displays all product access, but doesn't let you add any products to the user. This would not work as a bulk solution anyway.