You're reading the public-facing archive of the Category Theory Zulip server.
To join the server you need an invite. Anybody can get an invite by contacting Matteo Capucci at name dot surname at gmail dot com.
For all things related to this archive refer to the same person.
The proceedings of ACT2019 (ie. the one last year) are out! https://arxiv.org/html/2009.06334
Hurrah! It took a huge amount of time for EPTCS to process the submissions. (One reason: their style file doesn't work.)