Other forms: enrollments
When you sign up for something, like a new school, a medical trial, or a club, that's enrollment. Your enrollment in that art class is dependent on paying your tuition bill first!
In voting, enrollment means adding someone's name to the electoral roll after they've registered to vote. And in college, enrollment means the process of matriculating, or formally becoming a student at a university. You can also use this word for the total number of students at a school: "The preschool's enrollment is down this year." Enrollment is from the verb enroll and its Old French source, enroller which means "write in a register."