organization domain - xyz.com who had premium license, but their software development is done by another organization - 123.com, can xyz.com onboard 123.com domain users in the project
Yes, that is technically possible without any problems. But you need to add those users as users on your own site as well before they will be able to access your project. That implies they require a license on your site and you will have to pay for that license on your site. But they will be able to access your site and project with their own Atlassian account associated with their own email address.
I guess your question is mainly about the cost of the licenses, right? If so, that is what I tried to explain in my initial reply:
you can add users to your site with the email address of their company (their email address does not need to be associated with your company domain).
if you want them to use features in your site that require a product license, you will need to pay for that license. A product license is associated with the product, not with a user. Differently put: it is not because a user has "an Atlassian account", that he/she can access any Atlassian site around the world for free.
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