When cody arrived at the gym, it was relatively empty. she started riding a stationary bike at a leisurely pace. once a few other people sat down at nearby bikes, she began to speed up her pace and push herself a bit more. what is this phenomenon called?

This phenomenon is called social facilitation.

Explanation:

Social facilitation refers to one's tendency to improve his/her performance when other individuals are present. When the individual is alone, he/she has certain performance but having other individuals around tends to make this individual want to improve.

In this case, Cody is riding the bike at a leisurely pace but as soon as the gym gets less empty, she improves her performance by the sheer presence of others.