I upgraded to LR6 as soon as the option was available, and it seems to be a nifty upgrade indeed. However, when I exported a catalog and e-mailed it to one of my editing team they couldn't open it using LR5. Does anyone know of a way I can export LR5-compatible catalogs? Or is there a way to open LR6 catalogs using 5?
No it is not backward compatible—LR5 would have no idea about face tags, among other things and there’s no way to export a LR5-catalog from LR6. I’d guess Adobe is expecting everyone who wants to share catalog to upgrade.
If upgrading is not possible, and your editor is doing Develop module things, then what you can do is share XMP files. Right-click on a group of selected images or on a folder and choose Save Metadata to Files (or update the DNGs if you’re using those), then zip up all the XMP files and send those to your editor (along with the images) and have them do what they do, then send just the XMP files back, and you can select the same set of images or their folder and do a Read Metadata from Files to update your catalog with their edits.
If you’re editor is merely flagging and tagging then XMP files won’t help since they don’t store all the metadata, I don’t think. However if they are doing Develop settings, then XMP files might work.