There are no formal working groups under the new structure, though Contributors are welcome to form informal working groups as they please, though these are more likely to form around specific initiatives.
Initiatives may form from a requirement defined by the governance of the project, via requirements generated by new or updated use-cases, or through ideas generated within (or even initially outside) the overall GovStack community. This is especially likely when organisations implementing relevant systems in countries have ideas/experiences they think should be included in the specifications.