I think you should've be able to access vertex groups from the initial version of geometry nodes. Certainly, at this point, you can dynamically access (and set) arbitrary vertex groups, including loading the names of the vertex groups you want to access from a csv file, or otherwise synthesizing the names using string operations.
its actually funny, the first version of geometry nodes was actually more powerful in that way. it didnt provide many built in nodes, but the nodes it did provide were arbitrary property setting/getting. the current and newer geometry nodes system based on "fields" is way easier to actually work with but they don't give you raw access to properties. so like in the first version of geometry nodes you could write custom mesh normals just by setting the "normal" attribute with some vectors, but then from blender 3.0 to 4.4 you could no longer set custom normals in geometry nodes and they only added a set normals node in 4.5
I found the Entagma lab videos very helpful in understanding how to parse geometry with nodes. It is not obvious (unless you already get CLI pipes), but so worth it when things finally work... =3
