The best group prayer “activity” I have been involved in might be called a Thanksgiving Circle.  It is simple and has always lifted the spirits of the group. 

Stand in a circle.  The leader starts and briefly states something they are thankful for.  Examples: “Thank you, Lord, for the great weather today,”  “Thanks for getting me a job,” “Thank you for being able to pray together freely.”  Then the next person states a thanks.  Go around the circle ten times.  The leader keeps track and closes when done.

Listen to others’ prayers, and you will get more ideas of things you are thankful for.

You don’t have to hold hands, but if you do, it gives a person a chance to “pass” without saying anything – a hand squeeze means “please skip me this time.”

Don’t worry about repeats.

I have done this with groups of 5-10.  Not sure it would work as well with too big a group.

The current world record for the number of complete cycles through the group is 10.