How to slow yourself down (for playing nice with others) I have a road/touring bike that I take on longer rides with other folks. Sometimes other folks are riders of similar ability, and sometimes not. When I ride with people who ride slower, rather than changing my touring configuration (more upright bike, getting rid of clipless shoes/pedals) I'd like to just add some resistance to slow myself down/make myself work harder on the bike: this would make me go same speed as other folks while training harder than I would otherwise. I can imagine attaching an old-style bottle generator, but are there other good ways of adding drag/resistance to an existing bike setup?
Focussing on the "training harder" aspect of your question: 


A simple way for training hard while keeping your speed comparatively low would be to increase your cadence.

The easiest approach: Try switching a gear or two lower than you normally feel comfortable with, meaning you will have to pedal faster to keep the same speed. 

One of the advantages of high cadence cycling is less force on your legs, and therefore accumulated fatigue,  leading to a faster recovery after a long ride.

There is actually a nice GCN YouTube video on this topic: GCN: Improve Your High Cadence Cycling.


Focussing on the "playing nice" aspect of your question: 


You could offer to carry a bigger share of supplies and equipment. Collect those spare tubes, an emergency pump, patches, and maybe an extra water bottle, and the additional weight will make it certainly more challenging for you (and easier for your companions), especially if your tour includes some nice climbs.