The thing is, all the instructions and tutorials in books and on the internet will not teach you what its like to ride in a group.
You won't learn what its like to be be moving at high speed, boxed up against the kerb in the middle of a pack and how you feel about that. You won't learn how to close up a gap without leaving your packmates falling off the back. You won't learn to quickly identify the characteristics of your fellow riders signals. You won't learn when to break cover and dive down a hill then help string the pack back together again. You won't learn how much to let the rider in front get ahead on a tight bend or a sudden hump to avoid running at the back of them or thrashing it to get back onto them. You won't learn when you're having a good moment and inadvertently spreading the group behind you all over the countryside.
It's not just about signals and lines and all of this really is best learnt in the pack. Alas, now that you've covered 1500km in qualification, I have to break it to you that one of the best places to learn this is audaxing with ever changing groups and greater tolerance for varying personal styles.
I think a bit of pack riding with a local club is a good thing and most riders stand to benefit from it. Likewise, as Mr. BD says, you can't fully rely on the riding skills of strangers. People are going to have different interpretations of what you need warning of. Some riders won't care about riding through potholes or might expect you to follow their wheel so exactly that any discomfort you suffer for being over an inch out is your own fault. As always, the rule of audaxing is ride for yourself first.