Why not 1 alternator/generator per rider, combine and condition the electric power as required for the application. This is easily extensible, and partial capacity (say 1 of 5 people pedaling) operation still takes place at design voltage and parasitic loads are in proportion to the driving power. The electrical power combination would be simple junction boxe(s), or even just Edison plugs.
Mechanical combination to feed a larger alternator/generator does not have these plusses, and adds the concerns related to exposed shafting, power combination and other stuff.