Adding users to group fails


I run a custom LDAP query to find all the users with a specific title. This works and lists all the users I am looking for. I then select users in a specific office and then select the "Member Of" tab to add them to a specific group. I validate the DN (I use dsquery to get exact DN for the group) and the validation says it is good. I hit the "GO!" button and everything fails with the following error in the .xml log:
message="ADMODIFY.ERR - Exception has been thrown by the target of an invocation
No idea why.