A helper on co-op jobs is someone who joins a job.
The person who initiates the job is called starter. They are helping by planting the particular types of crops.
A helper may earn the Employee of the Month ribbon.
If there is a helper that you are not friends with, they will show up as a person with a blue head, and a question mark on their head (Similar to the one in Facebook). They will also be stated as friends of whoever invited them.