Golly what a lot of scaremongering!
Flying in 'loose formation' with a friend is some of the best fun you can have with a PPL.
The advice given by Fuji on this thread is pretty much spot on in my view.
The key things to remember in addition are:-
1) Don't get too close. Sounds really obvious but it gets tempting to try to get 'just a little bit tighter' onto your friend. Don't! It's not big or clever. Just dangerous!
2) Don't show off / dick about.
3) Don't contemplate doing this with anyone who you think might possibly try showing off / dicking about.
Assuming two sensible, moderately competent PPL holders doing this in good weather, not getting too close to each other and having followed Fuji's sensible advice - this really isn't dangerous.