a ) Perform a find to pull up a found set of trustee records on a trustee layout and loop through them using the data in these records to create new records in your portal's table with the needed match field values so that the trustee names appear.
b) Capture a return separated list of the needed data from the trustee table via a relationship or executeSQL and loop through the values of the list while creating new records.
c) Set up a relationship like this:
Meetings::AnyField X Trustees::anyfield
Trustees::TrusteeID = Attendance::TrusteeID AND
Trustees::gMeetingID = Attendance::MeetingID
gMeetingID is a global field. Using the OnRecordLoad trigger on your Meetings layout to perform this script:
Set Field [Trustees::gMeetingID ; Meetings::MeetingID ]
You keep the records correctly linked. You can now set up a portal to Trustees and include your check box field from Attendance in the portal row of this portal. As long as "allow creation of records via this relationship" is enabled for Attendance in the trustees to attendance relationship, this should work without the need for any scripting other than the single set field script to update the global field in Trustees.