Suppose the people are named Adam, Bob, Charlie and David.
Let's say the group with one person is made out of only Adam. The permutations for the other 3 are
Bob and Charlie, Bob and David, and Charlie and David.
We have 4 permutations if Adam is the group of one. If we repeat the same process for the others, we have 4 permutations for each one of them, therefore, the total amount of distinct groups is given by
We have 16 distinct groups.