We can't see your data so we can't advise in much detail. When you look at these invitation letters, can you determine which IEP record they should link to? That should give you a clue on how to match them. I'd look into setting up a temporary relationship that matches by existing data such as name and date fields, then add the needed ID number field to the table and use Replace Field Contents to copy over the matching ID number from the IEP table. Once done, you can take steps to rely on the ID number based relationship going forward and can remove the temporary relatiionship that was based on other data.