This question may be better in it's own thread if it is highly complex or contentious but here goes - As you have identified yourself as a group rider you may be able to give me an idea of how people *should* get into riding in a group as I have always understood that do do a good job, especially at speed, requires a lot more than just learning the hand signals. How does a group accommodate different capacity riders and for example. I think of group rides a little like a flight of ducks - Each one mirrors the actions in front a moment later so you would not a more powerful duck or one that corners differently in the group. And do riders in a proper (not loosley associated) have very similar configurations? Does a flat bar rider could respond differently timing wise and if so should they be riding only with similar bikes?
I am not sure that I am qualified to answer but as a shop group rider leader , I will have a go. PIHC run group riding skills courses and group leader courses both of which I would recommend.
Anyone wanting to ride in a group should ensure that they have had some training/education on group riding basics, it is not enough to simply be a strong rider. I can only speak for Garland's groups.
When someone asks about group riding at Garland's, they are encouraged to participate in at least one beginners session that is led by a highly experienced cylist who takes them out and explains/ demonstrates/practices the most important communications and techniques. This is done on very quiet streets. Furthermore, they are handed a comprehensive document that oulines in considerable detail the techniques, signals, terminolgy, comminications, ettiquette and skills required to ride safely in groups. A novice is not expected to master everything immediately but can refer to the document when required to refresh or improve their knowledge.
Once a novice feels confident enough (usually after one or two beginner sessions), they are encouraged to join a group. The groups are organised according to the average speed that the group will maintain through the ride, so most novices start in the slowest group, where there is more margin for error and on road coaching provided. As a rider becomes more confident and competent, they move up through to the faster groups. As a leader. I call riders on anything that is inappropriate and give them some advice/coaching. Often this is associated with having a sole rider mindset rather than a group mindset. I remind riders that we are like a large vehicle on the road and therefore require large traffic breaks to stay together and more distance to stop safely. The riders at the front and back of a group have the greatest responsibility as they comminicate to the group and other road users and make decisions on behalf of the group.
To answer your specific question about accommodating riders of different capacity. When riding in a group, the riders on the front do the most work as they punch the breeze, everyone else gets the benefit of being able to draft and therefore do less work. So to keep riders of mixed capability together, the stronger riders do most of the riding at the front and the weker riders tend to stay futher back. Of course if there is too big a difference in capability the fast riders will have to slow down as the slower riders would not be able to keep up. This is why it is important to ride in a group whose average speed is within your ability.
To ride safely in a group relies on good communications and everyone riding predictably. This is why it appears that everyone is doing the same thing (the flight of ducks analogy). Messges and calls are passed up and down the line, so that every rider is aware of what is ahead or behind.
It is possible for different types of bikes to be riden in a group and occasionally we have a flat bar rider or a even a fixie rider participate but generally having everyone on road bikes is best. Fixies tend to struggle up hills and are limited at pace, so would not be appropraite in a faster group (say over 30km/h average speed) as it would result in the group getting split. If the flat bar rider is strong, there is no real difference that I am aware of, other than the bike's limitations when compared to a road bike. They put the rider in a more upright position, so rider behind gets a good draft but the view forward is more obscured. Generally stronger riders tend to ride bikes appropriate to the task, so most group riders ride road bikes.