Slack signup can be opened as desired: https://github.com/rauchg/slackin
On that note, it looks like admin access has been locked down to only tkimnguyen (owner) and vangheem. That does seem like a closed-group approach, but it's a user decision, not something enforced by Slack.
XMPP-based would be great, but no one has built anything halfway as good as Slack on XMPP. Here I'm with @cewing -- I've tried plenty of chat apps, including gitter, and Slack wins.
One can host git repos on sourceforge or a dreamhost server, but Github makes it 1000 times more useful. I think same is true for Slack for chats.
I don't like that Slack is closed source, but the conversation export is complete and documented.
IRC/XMPP clients work with Slack: https://slack.zendesk.com/hc/en-us/articles/201727913-Connecting-to-Slack-over-IRC-and-XMPP
Slack is much more friendly to newbies than IRC .. avatars, inline images, attached files, emojis .. who does any client communication at all on IRC? All my clients have a Slack team.
I agree that arguing about using Slack is a waste of time. We all use many communication channels: email, Skype, WhatsApp, HipChat, IRC, Github issues, iMessage, Google Hangouts, etc. The Plone project uses quite a lot of Google Docs which is usually absolutely horrible except for the fact that they're very useful for certain jobs. We use twitter (and some even still try to have conversations on twitter).
I just pointed out a truly very useful tool that became available and said hey let's use this, in addition to all the other wonderful new things becoming normal, like github, jenkins, docker, AWS services, etc. We all use what we need and what suits us. We don't all have to use it. (I agree with @MrTango: certainly don't disable IRC.) It doesn't have to exclude those who don't use it: import archives publically.
In my experience, IRC is very wasteful: a lot of knowledge is shared there, but it has not been archived. There are many missed connections because the right people aren't online at the same time, and there aren't @notifications (that go to email if you are not online). If more than one conversation is going on at once, it becomes a hard to read tangle -- at least in Slack there are topic channels.
We should be careful in our choices and be aware of tradeoffs and compromises: the github issue tracker is not open source, AWS is a proprietary platform, etc. I do keep these in mind when looking at technologies and don't recommend any shiny new thing that comes along.
I believe for the people who like it, Slack will make developing Plone a more efficient and enjoyable experience.
Here's a page that makes some of the same points as I tried to: https://make.wordpress.org/chat/
The WordPress project is testing out Slack as our main real-time communication platform, replacing IRC and ad hoc Skype chats
Yes, there's only one thing missing --- someone building (something as good as) Slack on XMPP.
- Where we don't have to run a server,
- where everyone else is already using it, and have a team there (network effect, like github),
- where everyone is writing integrations with it,
- which has all the useful extensions integrated (e.g. syntax highlighting, long-form snippets, inline images & files) ..
I think it would be easier building on the JSON export, but for realtime export a Slackbot would be great. Perhaps the XMPP gateway can be used for that also.
It was very convenient during the training, see e.g. https://plone.slack.com/archives/training/p1444639657000007 (that id corresponds to a timestamp in the JSON export; the JSON contains a comment, a preview of the file, various metadata, and links to the file snippet, which would also have to be imported if desired. In IRC, links to pastebins are mostly considered transient, I think).