The simplist answer is maintain the same effort. So downhills you go faster and on up hills you slow. If you're new to a group let them know and tell them you will like their feedback. If you're in the lead the person behind can tell you to slow down or speed up.

Also watch and listen to what goes on between riders before you get to the front.

Most people are glad to let new people sit in on their lines and watch. Just tell them you're new to pacelinimg and want to learn.