If the guide is already married he would of course follow the army where his/her sentinel is.
If the guide is still unmarried most of the time they would introduce different sentinels that are considered great to try and match the guide to get him in their army.
Even though it is said that the army who knew it first would be given priority if their sentinels don't have the ability to woo the guide then it can only be their problem the guide would be in another army.
Guides who can do group guiding are really rare and they can do a lot as long as their mental strength can handle it. Even if they get married there would be no problem in transmitting their mental power to others and they can still effectively guide sentinels on a large scale.
Married guides will have a restriction on how they would guide others because their mental power is already bound with another sentinel.
