The PGDG doesn't really exist in the form you imagine. There certainly isn't corporate members or such. It's just a descriptor for all the developers together - it's not even a well defined legal entity.
Right, the members aren't the corporations; rather, the members are the developers, who happen to mostly work for the corporations that serve as vendors. That's what "corporate membership" looks like in FOSS.
E.g., in most Apache projects originally donated from a corporation, that corporation's employees usually still constitute a majority of the developers.
The corporation(s), in such cases, aren't sponsoring the project in any strict technical/legal sense. Rather, the project is steering and constraining the corporation(s) in their development efforts on their forks/extensions of the project codebase, determining through the project's core maintainership's decisions, what will be accepted/upstreamed from those corporate forks/extensions into the open core, vs. what will have to remain in those forks.
See: Redis vs. Redis Labs; CouchDB vs. Cloudant; Apache BEAM vs. Google Cloud Dataflow; etc.
E.g., in most Apache projects originally donated from a corporation, that corporation's employees usually still constitute a majority of the developers.
The corporation(s), in such cases, aren't sponsoring the project in any strict technical/legal sense. Rather, the project is steering and constraining the corporation(s) in their development efforts on their forks/extensions of the project codebase, determining through the project's core maintainership's decisions, what will be accepted/upstreamed from those corporate forks/extensions into the open core, vs. what will have to remain in those forks.
See: Redis vs. Redis Labs; CouchDB vs. Cloudant; Apache BEAM vs. Google Cloud Dataflow; etc.