Filter unrecognised USNs before attempting to add to Lookup groups

The sync process reports reports that it needs to add various people to the groups in Lookup every run. These people don't get added as no person exists with the matching USNs (due to Lookup currently only having a single USN per person).

Filter out unrecognised (by Lookup) USNs before attempting to add them. Log these as warnings.

Follow on from #5