The phrase "fellow inmates" refers to the people who are in prison or jail together with the person speaking. It means those who are in the same situation or facing the same circumstances. Full definition
Supporters of her release pointed to the work on behalf of fellow inmates she had done while in prison. (nystateofpolitics.com)
But as he prepared to leave prison at the end of his sentence, he promised fellow inmates that he would not forget them or his experience there, his biography says. (religion.blogs.cnn.com)
There he must learn to survive through organized death matches against fellow inmates. (kuriousity.ca)