Taking orders
Marines can take orders by having the player go up to them and press use on them (Unless the marine has User_IgnoreOrders). When used, they will cycle between following whoever ordered them to follow (In this case, you), wandering aimlessly, and standing still. Ordering marines with this method will also cause them to play a radio sound and show a little text over their head with the order they're now following.
NPC orders are only relevant when the marine is idle, and don't really affect their behavior when they are chasing and attacking things.
In addition, if the MVP is loaded, the radio can also order them, and will do so regardless of User_IgnoreOrders.
Being KAI NPCs, marines can also be ordered by map scripts using ACS_SetNPCOrder.
If a marine is a member of a squad, pressing use on them will make them also join your group, in addition to changing their order as above. If the marine is the leader of the squad, all the member marines (And KAI NPCs in general) will have the leaders' current order propagate to then. i.e if you order a squad leader to wander around, both the leader and all other marine members will switch to wandering around.