AntoniaTiger and kuroyume0161 are perfectly correct regarding the materials grouping for the original .obj file.
(BTW, AnthonyAppleyard has a utility "MakeObj" that can rename materials in .obj files Search in freestuff, or contact Anthony..)

However, when you create new groups in Poser using the grouping tool and save as a CR2, that does not affect the original .obj file. So all the additional info is then in the .cr2 file. And AFAIK, there's no way to rename/delete them in Poser, so a text editor is your only option...

I think.. (maybe there's a python script out there?)

Cheers,
Diolma