Answer:
If there are 5 people meeting after school, and they will form 1 group of 5 people if everyone shows up, then there are:
1 subset of 5 people (the full group if all 5 show up)
5 subsets of 1 person (each individual person)
10 subsets of 2 people (all possible pairs from 5 people)
10 subsets of 3 people
5 subsets of 4 people
To summarize:
Subsets of size 1: 5
Subsets of size 2: 10
Subsets of size 3: 10
Subsets of size 4: 5
Subsets of size 5: 1
In total, there are 5 + 10 + 10 + 5 + 1 = 31 possible subsets of any size from a set of 5 people.
So if 5 people may show up and meet as 1 group, there are 31 total possible subsets of those 5 people.