How to update different parts of the same Entry by different Users
Definitely multiple ways to do this but I think you're close. I wouldn't want to have multiple users hitting the same entry. Use Craft's power of relations to help here.

I'm guessing you're trying to great some kind of "rotary" or networking registration system?

You need 2 more sections: Guest Names and Guest List (name these whatever you want).


Guest Names is straightforward, it basically is going to be a list of every guest name available. You just need 1 field: the name. Craft's user ownership will take care relating this back to the User who created it and the Partner it contains.
Guest List is going to hold the master list. You need a couple more relational fields, one to relate back to tell which which event it's for (in your Events section) and another to "hold" the names of the guests attending.


Now it's just a matter of wiring it all up. You're going to want users to post to the Guest List and Guest Names sections. It's up to you how you want to handle the validation on the front end as to whether a guest already exists that's "owned" by the User.  

The beauty here is also if you do multiple events and the same people attend the events, when someone registers, you can simply pull up their prior Guests and offer those up as guest choices too. Again, I'm just speculating.

When it's time to pull out a guest list, just query your Guest List section based on the Event you want, loop through the event, and grab the names for each attendee.