Adding users froma trusting domain

[Win]

Can you help?? Here is the problem. I have three domains in my business.
My domain is trusting two other domains. I want to give users from the
other domain permissions on my folders in my domain. Is this possible via a
command prompt. If so how? How would I add users from domain x to a group
in my domain. I know this is a long question but would be grateful for any
help.

Thanx