I also had this problem some time ago, when I started making missions. By trial and error ( I didnt know about OFPEC at that time :'( ) I found out that if you place a WP Get In for the group, and synchronize it with the WP move of the helicopter ( which is in a different group = alone ), they will load into the helicopter. Now if u place a WP for the heli to Unload Transport and WP move somewhere else, you will have what you want. The infantry group will then follow their next WP.
Tomasz