Funny how this has never come up for me until now, but... In K2, is there a registered user front-end editing functionality like there is in native Joomla articles?  I need to have several people contribute to different categories and I would rather not have them logging into admin in order to create articles in K2.

Just enable frontend editing in K2 parameters and then setup the groups and add users to the groups.

I get that, how do they get an editing menu?

None of us can read minds.  The more information you provide the better the answer you will get.

Enable the k2 login module.