The best solution for six persons is 19 trips. One of the solutions can be described as follows:
Number the persons as 1, 2, 3, 4, 5, and 6.
Only 4, 5, and 6 can stay on either bank alone.
Two persons can stay alone together only if their sum is no more than 6.
Three persons can stay on a bank only if 1 is on the same bank as 2 and 4 is on the same bank as 5.
The answer we intended for the bonus question is that Frances E. Allen (read the number as ASCII string) was the sixth president of IBM Academy of Technology, but Alper Halbutogullari found out that she was also the sixth in the list of 19 people that are both IBM fellows and ACM fellows.
Bert Dobbelaere was the first to find a 81-trip-long solution: 7F9B939F4FFEF7FC91DF7F7FFFF9EC6B.
There are several ways to search restrictions which yield a solution of at least 73 trips. Thomas Huet (thanks!) wrote a nice blog post about it: https://thomash.fr/2019/06/14/ibm-ponder-this-may-2019.html
Alper Halbutogullari (thanks!) sent us a visualization of one of the 81-trips long solutions (6dfeb51fc71ff77e1776bfef7ffaf8c9):