There was a time, when cycling was far less popular, that a novice would learn the craft of cycling in a club environment. This still happens pretty much in racing, where it is absolutely critical that riders possess a minimal level of skill as well as knowledge of others' expectations in a group. But I think it happens far less in social cycling. This is a shame, because a significant amount of important learning (as well as some culture, customs, and tradition) do not get passed on.
I started my cycling apprenticeship back in 1972 with the Berkeley Wheelmen, and the time I spent on those Sunday morning training rides was absolutely critical and extremely beneficial. Today I feel far more comfortable on a 200-meter velodrome at 50 kmh+ in a group of racers within inches of me, on a bike with a fixed gear and no brakes, than I would on the typical recreational ride. Granted, I know if anybody bumps me, I'm not going to be the one going down (unless he blindsides me from behind), but a competent cyclist should be able ride close enough to brush arms with the rider next to him, and the skill level of most social cyclists tends to make them overly concerned with avoiding such close riding to the point that you get situations like your "nimble" rider, who was definitely too far out in the road. If he couldn't compact himself right up against your tandem (tricky on a winding road), he should have temporarily moved into single-file, and in a competent group, the leader would likely have called for a single file in that situation.
You need to find a club with good leadership. As an example, we often ride with the Evergreen Tandem Club in Seattle, WA. They provide courses to train ride leaders, and I don't think they'll let you lead a ride until you've taken the course and been certified by their experienced club leaders (I think that's their policy, as I've never had occasion to lead any of their rides!) If you can find a club like that in your area, you need to ride with them. It will greatly improve your proficiency in group situations.
The last thing you want to do is ride with a Critical Mass group. Those guys are complete idiots!
L.