Originally Posted by
upthywazzoo
20 meters? Really? Even with no wind, that's way too far back. At one bike length off a field, you're feeling the wind. At two bike lengths, you'll be struggling. At three, you're dropped.
And as other said, do just jump on a random person's wheel. It's not a cool move. If it's a big group, that's a bit different, though it is generally best to ask, "Hey can I join you?" or "How far are you going?" If they don't seem friendly, that means they don't want you to join them but are too nice to say that out loud. Don't be offended if they don't want you to ride with them.
As for how close to ride, that depends on how well you know the wheel in front of you and how close everyone else is. If you and a friend are good, you can ride really close. I had a riding partner who was so steady, I was comfortable to put my bar over his rear wheel - we were overlapped due to a cross wind. If the group is all riding with about a foot between wheels and just a few inches between bars, that is where you should be too.