You work with Reformists constantly, if you are a true Activist. It's kind of unavoidable. It's necessary to achieve concessions for the working class under Capitalism and reformists tend to work for concessions. Within movements like the trade union movement, the feminist movement, and the queer rights movements, the majority are not revolutionaries but will seek concessions favourable to oppressed people.
Pan-leftists are not reformists but revolutionaries, however, we do have to co-operate with reformists when fighting for concessions and not devolve into sectarian idiots.