As said it is best if everyone agrees to ride together. If I am out solo and someone wants to draft it does not bother me as long as they announce themselves. I have done so on occasion when I am solo and asked to jump on the back when being passed. If you ask most people are Ok with it. I have had someone draft me for as much as 8 miles without a word which is somewhat disconcerting.

If I jump in with a big group I generally don't say anything since who am I going to ask if I can jump in. Most of the big groups don't even know everyone in the ride. Not sure if that is breaking a rule but I have done this on many occasions with no one complaining.