Simple fix for a environment where you have a one way forest trust between an environment that hosts a SharePoint site and several other forests where you would like to use those accounts to login to the site. After establishing required trusts and domain settings the only SharePoint setting that needs to be updated is the people picker. The people picker only looks up naively to the local forest. The steps to get this working are as follows:
1. Create service accounts in each other forest that will be used to
2. Execute the following command, you will need to adjust it for your environment in this sample i’m adding 3 forests.
stsadm -o setproperty -url https://intranet.cotoso.com -pn peoplepicker-searchadforests -pv "forest:ad.acme.com,acme\sp_adtrust,password; forest:northwinds.local,northwinds\sp_adtrust,password; forest:ad.microsoft.com,microsoft\sp_adtrust,password"
Note: you always need to add ALL the domains if your updating just one it will override the ones saved. So make sure to include existing ones if your just adding one.