Re: Creation of the user otherdomain\username cannot complete because the user is not a member of any the mapped groups

U correct Trust is OK and registery not required.

Try adding the group by Distinguish name insted of simple name.(u will get it from AD team)

I know u able to add the Group succesfully, try adding the group in Distinguish name format and then try creating the AD user manually for them.

let me know how it works.