What term do psychologists use for the phenomenon that occurs when people are less likely to aid a person in trouble if there are other people around who are also potential helpers?
This is known as the bystander effect.
Bystander effect is where people are (like you mentioned in your question!) less likely to help a person if there are others around.
A potential explanation for this is the diffusion of responsibility. As the number of people increase, the responsibility to help a person in need, diffuses more to each person. This means the more number of people, the less an individual of that group has responsibility! Thus, the less likely a person will help as they are held less responsible (due to a larger amount of people).
However, for instance, if there were one person around the person in need. For eg. person B falls suddenly and there is no one except person A around. Then person A would have all of the responsibility to help person A up. Consequently, person A is much more likely to help person B.
If you do want to read up more, here's a link: https://www.verywell.com/the-bystander-effect-2795899