The Reinforcements will follow you where ever you go, and they can't be attacked; depending on the zone and your level however, you could, so be cautious.
There are three types of behaviors for the NPCs, which you can confirm by checking their hair color:
Brunette: She is concerned for the other soldiers at the checkpoint, and will occasionally run off towards it after announcing that she is worried.
Redhead: She thinks there are enemy scouts everywhere, and will occasionally freak out and run off randomly looking for them.
Bleached: She is very impatient and will sometimes run off towards the stronghold without you. This is usually a good thing, as you then only have to babysit the other one. Closer to the stronghold she seems to forget where it is though and you will have to guide her the rest of the way yourself.
Each NPC can be made to follow you again simply by talking to them. They will also never run farther than /say range from you, and will make an announcement when they stop moving, so it's possible to deliver them one at a time, returning for the one you lost after escorting the first. If one happens to run off and has what seems to be too much of a lead on you, it is actually better to NOT chase them. Instead, they will stop once they are just outside of say range. Otherwise, they will continue to run so long as you are within /say distance and have not caught up to them. When you start the mission they will already be following you, so you do not have to talk to them until they try to run off somewhere you don't want them to.
If you have the bleached NPC, simply let her run ahead of you and focus on the other NPC. Eventually you _will_ catch up to the bleached hair NPC. Then just talk to her again to get her moving (or leave her be while you finish escorting the other NPC if you are near the stronghold). If you have the brunette NPC, one strategy is to simply let her stay at the checkpoint (where you pick her up) and come back for her after escorting the other NPC. No matter what, you are guaranteed that at least one of the bleached or brunette NPC is among the two NPCs, so there is always a winning strategy when soloing.
The chance of them running seems to be based on distance and eye-contact; maintaining a reasonably close gap between yourself and the NPC, and turning around to look at her, seems to greatly lower the chance of her taking off. You can keep one of them targeted to save time when they do run.
Once the NPCs reach the stronghold, they will make an announcement about it and stop moving, and will no longer follow you when you talk to them. Make sure this happens before you speak to the arbiter or you will not complete the mission.
This mission can be done even in zones that already have the maximum fortifications; what it does in this situation is not currently known.
This mission can be done as a party of up to 6 members. Only one member needs to have the mission active. All members, presuming they all get a chance to have an NPC following them at least once, will be awarded the same xp (the reward is neither split nor reduced), however only members with the mission active will receive Allied Notes. For maximum xp, having only one member get the mission at a time will greatly improve the amount of xp your group can gain from this op.
Only one group can do any tier of this Campaign Op at a time so you may have to wait for another group to finish. In this case the Gate Sentry will say "The troops you are to guide to the stronghold have not arrived yet. Please wait a little longer."
If you are at a low level and depending on the zone, you may want to have Sneak up to avoid aggro.
Be careful in Sauromugue: if a mithra runs away, she might jump up on the ledge with the Maw and become unreachable, causing you to fail the mission. Keep a careful eye on the brunette: if she jumps up that cliff, follow her until she hops back down before you run out of range. And keep the redhead far away from that ledge. (The bleached mithra shouldn't give you any trouble with this ledge).
Federation reinforcements have been dispatched to assist in the defense of our nation's strongholds. You are to travel to any checkpoint garrison under Windurstian control, and escort the awaiting soldier to her final destination.