I've been dealing with enterprise portal but using cross domain trust when accessing the enterprise portal website. When adding I tried to add users from other trusted domain the Sharepoint didn't recognize the user I tried to add.
To be able to add other trusted domain you can use below Sharepoint powershell:
STSADM.exe -o setproperty -pn peoplepicker-searchadforests -pv domain:"trustedDomain" -url "http://WebApp/"
After execute the powershell script you will be able to add other domain trusted.
