Quick disclaimer: I'm unsure if we'll be doing any multiplayer or even use Core.  I'm just curious for suggestions if we did decide to try it.

It's pretty odd in my experience to see a jam to offer multiplayer as a challenge.  It's pretty cool, don't get me wrong, but in only a 3 day time period I would expect to have trouble playtesting a multiplayer game even if it was built in Core where multiplayer is already implemented.

Multiplayer games can introduce a lot of relevent design variables that need to be tweaked to maintain balance and fun, but in order to find what needs to be tweaked you need people who aren't you to play it (and enough of them that your game still functions correctly).

Any tips on how to get enough different people to playtest a multiplayer jam game?  I'm interested to hear your thoughts.  Thanks!    

if you set on making a multiplayer game it would be good to at least know what you are doing, for example unity's built in internet multiplayer system has been depretiated, so you need to make your own or use a third party option.
playtesting-wise, if you have multiple screens its alway good to have a recent build of your game open so you can test stuff. sometimes even if the build and in engine game are different you can still conect and test out(though the build will not have the same functionallity as the in engine, and you would have to build to get that working) 

if you want to do a local multiplayer thing thats great, though most people probably dont play gamejams submisions with their friends, so if you do add local multiplayer its probably best to do a cooperative sort of thing  where you can play it 2 player but its not nessasary(that way more people can actually experiance your game)

3 days to do all this probably isnt the best idea but if your dedicated and end up doing it, im certainly interested in anything you might come up with