Better support adding participants after the first participant list upload

[arsalansufi]

yeah - I have to assign the jurisdiction after I add the email. I wonder if, in the future we can have all of the counties in the participant list without logins and then just update that way... it's been a bit of a pain keep tracking of this participant list without having logins for each county from the start. I have a master copy, then add new logins, then copy the master copy, then delete the counties with no entry from the copy... repeat.

https://votingworks.slack.com/archives/C022U9ZN7EV/p1710864267779379?thread_ts=1710863633.124899&cid=C022U9ZN7EV

[ginvdr]

May want to consider this with this ticket: #1779