It's possible that there are too many players online. My game used to crash when that happened. After a while it just sort of went away...
If the game keeps crashing when other people enter the map, it's possible that they haven't downloaded the files correctly/improperly placed a download/mesh/texture/file.
If it's a private map we're talking about, they might have messed some things up in the process of creating the map.
You could try uninstalling the game, then re-installing it- sometimes a few things get lost while all of the updating and downloading occurs.
The only other thing I can think of is ...possibly the person's computer doesn't store enough available data space? I'm sorry if I'm not much of help, but there isn't anything else I can think of.
As Hooli mentioned above, you could always try deleting certain files to see if it fixes anything.
Good luck with the game.