I had always thought that the rules I'm about to outline were actually part of the core rules for Mythras, but it turns out that they weren't and I'd only imagined that they were (or else Loz and I couldn't find them when we glanced through the rules). I used these rules at Lozcon this weekend (Lawrence Whitaker's weekend roleplaying convention held annually at his home) and they were a hit. In hindsight, they're a simpler version of the teamwork rules I came up with for Openquest.
Whenever characters want to assist one another (i.e. they are all searching a room together; or two smiths working on the same project), they designate a lead. The lead is usually the group member with the highest skill. The lead is the character who will make the roll. The lead may have up to three assistants. Each assistant must have a score of at least 25% in the relevant skill. Each assistant reduces the difficulty grade of the roll by one. If the lead fails or fumbles, both the lead and the assistants suffer the consequences of failing.
This tends to simplify perception and sneaking rolls tremendously.