When someone suggests an edit and provides their XMP, how do you actually import that into DT? Can you do that without overwriting your edits (i.e. the helper’s XMP would be a new duplicate)? If several people provided different XMP, can you import them all?
DT3.8 if that matters.
Make a duplicate of the image, then use the load sidecar function from the history stack module: darktable 3.8 user manual - history stack
You have to duplicate in darkroom, then go back to lighttable to load the sidecar?
Append mode seems to be the default operation with load sidecar. It seems you want to use overwrite mode to get only the other person’s edits. I didn’t do this the first try 
Five people tried edits and provided the XMP. It’s getting hard to keep track of which person made which edit. I shot in RAW+JPG. I started an edit of the RAW. This is version 0. I’ve loaded 2 of the 5 edits which show up as versions 1 and 2. I tried tagging them to identify the person that made the edit. That feels clunky. Is there a better way?
Not that I’m aware of.
You can duplicate in lighttable too: select image and then ctrl d
In lighttable view the setting is sticky: Change it and it stays that way until you change it again.
Not sure if it is better but you can use the metadata editor in lighttable view and use the title or notes field. This module does have the ability to set what is(n’t) shown (use hamburger → preferences…).
One other thing: Edits made with newer darktable versions (often mentioned as 3.9 and/or development) might not work if you import them in 3.8. Those edits might have stuff in them that is not yet available in 3.8
I use the metadata editor in the lighttable to keep a track of the duplicates and I find this particularly useful when I’m looking at the play raw edits to learn how others use darktable.
I use the duplicates to load each users XMP then use the title in the metadata editor to add their name.
I like @Phil_Smith’s suggestion, however, when I click duplicate in lighttable, it delete all of the titles. Not just the new one. But all of them. I didn’t realize this until 4 or 5 copies. I was titling the imports with the person’s name, but all of the titles are gone.
3.8.0 bug? Windows bug? Intended functionality 
?
Not sure how you go about doing this, but on Linux it looks like this (darktable 3.8):
I hover over the duplicates in between setting the title field and you can see that they stay as set by me (original, user 1 and user two).
I turned on screen capture to make sure you can see what I do with the keyboard/mouse.
I confirm mine works as the video by Jade_NL.
Ubuntu 20.04, darktable v3.8.
Maybe I didn’t press the Enter key after entering the title. Your screen capture showed that keypress, so thanks for showing that.
Thanks for putting up with me. I’m new to RAW and DT. Lighttable mode brought me to the party. I’m still getting my feet wet.