What exactly do you mean by "split them in half based upon the first letter of their last name"?
the first find will find members Anderson through Magillicuty. The second find will find members Newman through Zwernemann/ A thru M, then N thru Z. Based on last name.
Enter find mode
Specify < n in the last name field.
repeat this but specify > m
That easy. I thought it would need some sort of "Length" calculation. Duh! feel silly here.