open the File dropdown menu in the top left corner of the editor with the map selected
select "Create a new difficulty"
enter a diffname, select your difficulty settings, make the map
select "Open Song Folder" from the File menu
take the .osu file that corresponds to your diff and send it to the person you're making the guest diff for
what they'll do on their end is simply just open the map folder as above and put the file you sent them in that folder
as for collabs, the most common way to handle them is to agree on who maps which parts and then just send the .osu file back and forth, each collaborant mapping one part at a time
another way is that everyone maps their parts on their own and then everything is
compiled into one map at the end, but that's even more prone to incoherency than collabs usually are. it also tends to take some effort to make the transitions between sections make sense