Both the Files and the Photos section allow one to enter any character pretty-much in the folder, album, or file names.
If one does a Files or Photos export however, those special and non-acceptable (at least for Windows) characters are carried over when the folder/album+files/photos structure is replicated into a regular folder+files structure, and that causes problems when one tries to extract the zipfile contents. In Win7 at least, if one selects the zipfile in Explorer, the folders+photos do list with the illegal characters but they cannot be extracted. Attempting to use Winzip fails as well, albeit it's an older version, maybe new versions have some kind of auto character conversion option, I don't know. I haven't tried any other extractor but suggestions are welcome.
I guess this would work fine if the export was only used/intended as a backup zipfile for recreation import (only by you) into groups.io. But for us it does make the idea of exporting files/photos for doing an en-masse import (i.e. moving) to a subgroup or another group pretty-much impossible for folders/albums/files that include those characters, one can only do this saving one file/photo at a time. (since manually saving a file/photo with those characters in its name works in this case because there is auto-remapping of those characters into a dash)
I am in the middle of renaming albums, photos, & files in my groups to weed out those bad characters and I'm having way too much fun (tongue firmly in cheek), so the suggestion would be to do the same auto remap-to-dash when creating the export zipfile. Or alternatively, maybe have a character remapping ON-OFF option in the export screen, if it would be beneficial to have both remapping options available.
As a related item also, the owner's manual doesn't mention this limitation in the Export section, something may be needed there, I guess depending on what's decided for this issue.