It is much easier to figure out harmonies when you know that basic rules that the arranger is using, than if you just listen to a recording and try to guess--I am not familiar with the Poozies, but I think that you might try to contact them--you would be amazed at how easy it can be to find people on the internet--

The old rule of thumb for four part harmony is that the melody comes first, then the bass, and the the other parts fill in--but there are other options--some use a lead vocalist, and have the others sing block chords, some use broken chord--It helps if you try to figure out basically what is happening in the music before you try to plot out the notes in each part--

This is a difficult project that you have set out for yourself--Writing and performing multiple voice music is one of the hardest things that you can do, musically--partly because everything has to be exactly right, or the whole thing falls apart--Good luck!!