Signed up for the Wiggle Ashdown Sportive. Some tips required

Try and get out with a local club beforehand - one that matches your ability. Great way to make new friends as well as become comfortable in group riding.

British Cycling, LBS or google are good places to look, and most let you try a couple of rides before joining.

Naturally you will find groups that you can tag onto. Riding at a pace which is above your comfort zone will be apparent because you won't be chatting with the group. Stay close 1 - 2 ft off the rear wheel for aid in drafting. The downside is that you need to be fully aware and have sharp reactions. Plus is you can add several mph above your normal pace without tiring. Hills will find you out in group riding, if you're not a quick climber you will lose the group end up solo until another group comes upon you, tag onto back. My first sportive I spent 60 miles riding in two groups sharing the lead, I spent 10 miles being drafted by a pair of fast riders above 20mph. The rest solo.

I did three of the wiggle sportives last year (I had a long time off the bike, got my first road bike around 16 months ago), they were the first and only "group" riding experiences I've had. Normally I just go out on my lonesome on the weekends when convenient.

On the first one I ended up riding with 4 guys who rode at my pace for the first 40 miles, we made great time and while I had to learn fairly quickly what the various hand-signals meant - it seems that just shouting is acceptable in most cases ("clear" at junctions, "pothole" if you are going to swerve to avoid something).

I certainly wasn't comfortable taking my hands off the bars at high speed in close proximity to others at very short notice. I don't know if only the guy at the front is meant to warn of potholes, but it struck me as a little risky.

Ideally you want to be about 1 wheel length behind people from a drafting PoV. If that's too slow for you then just move past them and try and find another group. Be sure to take turns at the front though, from what I've read the person at the front expends about 30% more energy than those drafting them.

On the second two sportives I pretty much didn't see a soul apart from at food stops. It seems I left fairly early, from what the folks manning the food stops were saying, I was the second person they'd seen that day. It essentially turned into a solo ride.

Just an FYI, my normal solo average speed is around ~16mph. That first sportive when I got a group moving at my pace I averaged ~20mph - so much faster. Now to be fair, the first one was the spring, new forest sportive, and is pretty much flat, whereas the Chiltern and Ups and Downs ones (Dorking) are considerably more hilly.

HTH

There are various signals that groups use to warn people at the rear of hazards coming up, you can probably find them on Google. Give yourself a bit of room and cover your brakes, if you're too close and overlap with the wheel in front and you touch it can be very hard to not crash. Listen out for calls as well, usually as you approach a junction.
Best thing to do is to try and get some group riding in beforehand.
I haven't done the Ashdown sportive, but do know the hills and they can be very tough if the wind gets up.
Also try and get chatting and take a turn on the front if you're feeling strong.