The Reinforcement will follow you where ever you go, and she 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 NPC, which you can confirm by checking her 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 tends to make the mission much easier as you don't have to babysit her as much. 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.
The NPC can be made to follow you again simply by talking to her. She will also never run farther than /say range from you, and will make an announcement when she stops moving, so don't panic if you can't catch up to her. When you start the mission she will already be following you, so you do not have to talk to her until she tries to run off somewhere you don't want her to.
The chance of her 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 her targeted to save time when she does run.
Once the NPC reaches the stronghold, she will make an announcement about it and stop moving, and will no longer follow you when you talk to her. 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 the 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.
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.