If you cannot party with them, its opssible you already have some people in your party so they cannot join. However, you should be able to use the load into entity option at the bottom and choose a destination (a city should be a valid destination) and they will move out.
Thanks for the swift reply. Unfortunately I can't see the squad as selectable or even as individual units in my party screen. When I'm in the movement screen I can expand the selection under the npc heading and see individuals. In the party screen however, I can't do this. Am I making any sense in my explanation?
There's one thing that could explain it (unless it's a bug):
Owner created a squad out of these NPCs (more likely if they are military) and as long as he has them in personal squad no one else is able to command them.
If owner is active and that's the case, it's still possible to resolve it through IC means. (he just needs to go to his Party screen and click disband squad)