I also fixed this problem by renaming my profile. After the rename the game refused my attempt to change it back, but changing the name of the folder and files worked.
Nov 21 2016
May 10 2016
Sorry for the 4 months delayed reply, I must have missed the initial email update. I do not experience the bug any more, and even though the bug is elusive, I do not think it is present anymore. Especially considering there has been no activity on this report for the last months.
The problem persists even if you sync the infantry group to the chopper. And remember the reported bug is that you can't make them mount occupied vehicles, not empty ones.
The second method you describe just makes the group a motorized/mechanized infantry group. Motorized/mechanized infantry groups will mount their group vehicle when you give them a "get in" waypoint, that's not the issue I addressed in this report.