I just opened GH inside Revit for the first time. I referenced objects by family types successfully. I need to first get a specific parameter values (this parameter was added from shared parameter file), modify it in grasshoper and SET the same parameter for each instance.
Unfortunately it seems that the components have been updated and i cannot rely on examples from official site:
Generally it does take a string as a Key. There is a utility on the guide above to Find Built in Parameters. That might help identify the Keys to get to the parameter values. There is a good section on reading the values too. Between those two I think that may get you going.
The whole GUID issue is an interesting one. I know that it looks like a GUID, but I believe in Revit it is not actually one. @eirannejad can explain this much better if it is important in this case. But, I think the correct string for the Key will make this work.