How does the Group Provisioning locate the members that exist in Exchange when Provision Groups?
The Group Provisioning searches for the users based on the targetaddress value in the groupstoprovision.txt file.
Groupwise Migrator for Exchange will in this case search for firstname.lastname@example.org to see if this user exists in Exchange. If this address is a valid email address for a user in Exchange this user will then be added to the membership of the group.
If this address is not there please add this address, or change the address in the groupstoprovision.txt file.