GAL synchronization should be in place between these two forest (two Exchange organization). If this is in place, you will have cross-forest mail contacts for recipients from the remote forest. Than you can add the relevenat cross-forest mail contact
as member of the DG group.
Exp: ADATUM.com -forest 1, dg1@adatum.com DG1
CONTOSO.com -forst 2, dg2@contoos.com DG2
After you set up GAL synchronization with Forfront Identity Manager (FIM) you will have cross-forest mail contact
dg1@adatumc.om authomatically created in forest2. This should be added as member in DG2.
If you don't need GAL sync and FIM, simply you can create a mail enabled contact in forest2 and set
dg1@adatum.com as external address. Than add this mail enabled contact member of DG2.