Get AD Group Member - Different Domains
I found using get-adgroup -properties member to be more reliable for returning all objects.
To get objects from different domains I do the following:
get-adgroup -properties member | select -expand member | get-adobject
From this get the distinguished name of the object, this will tell you what domain to search, so in the end you should have something like:
get-aduser $user -server $foundDomainDC