Welcome, @Muhammad1,
If what you’re saying is you have user1@myorg.org and you don’t want to pay for them but let them have access as a Guest instead of as a Member, then I think they could edit their Asana settings to add their non-org-domain email address, like user1@gmail.com.
Then change all Asana task, project, etc. references (assignees, collaborators, owners, rules created, etc.) from the old user1@myorg.org email to user1@gmail.com.
Then deprovision the old email address; see also:
I might have something wrong/missing here. Maybe another @ambforumleader or @pforumleader could correct or improve upon my answer here?
Thanks,
Larry