AD setting with child domain

[keywtao]

I have joined parent domain (company.com) and assign a shared folder within this domain. Today, i would like to add a user in child domain (child.company.com) to this shared folder but i cannot find user from this child domain. What AD setting should i add and allow to do this? Thanks.

I.E. Parent Domain = company.com
I.E. Child Domain = child.company.com

Kevin