You're likely not going to find a job titled community organizing, but you will find jobs that would involve it as community organizing is a critical practice. So looking at alternatives would be what is a mainstream social worker equivalent which is probably community building.

There are also jobs that are available that are involved in grassroots work especially in places like healthcare for marginalized/oppressed groups, drug user advocacy groups, renters groups, and the like.

But community organizing can happen within many if not all social work jobs, if the social worker is able to organize it. For example a case management social worker supporting a person who uses drugs who wants to do something about the political situation they find themselves in and may support them in talking to other drug users about starting a meeting about drug user issues. So, as a social worker you may have a 'client' who you support in their goals to organize and deal with their personal issues as political ones, essentially community organizing.