Granting access to users in a trusted domain external to the fores

[Jesse Schuchart]

Client has other business units that have seperate trusted domains that do
not reside within the forest.

I am thinking the following should work:
Create universal groups on the external domain for these users
Add the universal group to a domain local group on the forest
Import the domain local group

this should work, but I am getting an error stating:
The specified domain either does not exist or could not be contacted.

Does this configuration for the groups sound correct and viable within
Project Server?