Lets just say that my group is fighting a group of black Templars that are in full plate armour. How would you work the fact that the suit of armour is slow and clumbersome.
As a GM you basically don't. As a player you can declare or assess that the Templars are hindered by their heavy armor to gain a free tag for +2 on them and consequential invokes though the spending of fate points.
Is it an Aspect of the Armour do players make a decleration that the helmet restricts vision. How does that work as a reaccuring fact. eg one player declares that said knights helmet restricts his vision to help his sneak past the knight. Another player wants to do the same. Can they both declare the same aspect? or does the second player have to tag (which seems a little unfair) and spend a fate point. Is there such a thing as a continous situational, or free taggable aspect.
Does that make sense?
As far as I understand it, the knight might defend against the declare/assessment roll with an appropriate skill like alertness, athletics, might or the like. If the player fails the assessment, he doesn't know of the hindrance the armor provides and don't get an aspect to tag. If he fails at a declare, he might think that the armor provides hindrance when it actually does not and get slammed in the face by angry knight that is quicker then anticipated.
In your sneak situation I'd handle it as a team effort. There are rules for teamwork. If you want to make it simpler, you could just let anybody declare and tag on their own or allow everybody a free tag. Don't over think this stuff. Most times guards are meant to be overcome by the players. And think about this: if anybody fails their declaration, their PC might think they outwitted the guards, when they actually might not have and bang! The alarm is raised. Fun!