I think you want to have a list for the first 6 months that may have 50 volunteers listed by month, and then the next 6 months listing with another 50 volunteers, some of whom may be the same volunteer as listed in the previous 6 months.
You can't do that because you only have one record for the volunteer, so it can only be placed in the listing once. You could of course find the first 6 months and then separately find the second 6 months, but that is too limiting.
You need to create another table ('File' in FMP5) called VolunteerDate (or something) with one record representing one volunteering 'event' - one record for one volunteer for one event (month). You can still have the table of Volunteers showing one record per volunteer. You can have a portal (new feature since FMP5) to show all the Volunteering Events that volunteer has signed up for.
You can have a global field on the Volunteer's record that has a pop-up value list of the months. Beside it have a button 'Log this volunteer for this month' and when you click it it creates a record in the VolunteerDate table for that volunteer, that month.
Yoy can then run the listing you want from that table.
You can have a portal (new feature since FMP5) to show all the Volunteering Events that volunteer has signed up for.
And Sorbuster's excellent solution would have also worked in FileMaker 5 had you structured your data in that fashion. An alternative to the global field is to set up a field directly in the portal for logging a volunteer for working that specific month. That can be a little bit simpler as you don't have to use a script to create your records in the portal,
"ah actually, we've had portals since the release of FileMaker 3" We did? I almost can't believe that. Thought Lookups were the only connection until about 5. Shows my memory doesn't span the centuries too well.
Thanks to you both for the suggestions. I'll spend some time trying to get myself up-to-speed on the new Filemaker and see if I can get them implemented.