In actual warfare, the lowly Infantry trooper will hitch a ride aboard anything that will carry him. They will ride behind tank turrets, cling to door mirrors, ride atop shipping containers, etc. In Ogre terms, this should mean that any armor vehicle (medium tank, missile tank, mobile howitzer, heavy tank, etc) may carry 1 squad (up to 3 guys) of armored troops. They may mount or dismount during the movement phase as their only movement. While being transported, if the unit carrying them is destroyed, the troops are as well. The troopers may not fire while mounted.
Addendum: it has been brought to my attention, and rightly so, that 3 guys is actually 3 squads. My confusion came from the counter of a strength three infantry, being three single squads stacked together. Each armored vehicle should be able to transport only 1 squad at a time.
PS: thanks Aramis for the heads up!