An easy way to implement this would be a "toggle switch" key combo that the owner of the units could push with any units selected, which would allow any ally to select and control them. There would need to be some visual indicator that the units were in this state (visible to the team only). The owner's commands would always take priority, otherwise the unit(s) would follow the last command given by any team member. The owner could take exclusive control of the units back at any time by toggling the switch again. In the classic situation, cavalry could be toggled into this state and moved to the far side of the map from the owner, who messages to a team mate, telling him to use the cav. That team mate could control them as long as they were toggled as "share." Using this model, so could any other team mate.

Kas brings up a good point in that this would benefit experienced teams most of all. However, I don't see this as a reason not to try it. I think it's a good thing, to encourage integrated team play with features that make it appealing. For me, killer team play was the biggest rush of all in MP. This feature could be enabled/disabled on the Game Setup screen by the host, which would provide some control over its use.