The Pico-8 community uses Slack for real-time discussion and collaboration. The "pico-8" Slack group has active users and multiple channels.
To join this Slack group, sign up for Slack, then enter your email address in this form: http://catgirl.goddess.moe:3000/
You will be invited to the Slack group automatically.
The Fantasy Consoles Discord server has a Pico-8 channel, as well as channels for other fantasy consoles inspired by Pico-8 and otherwise. The community runs regular game jams: http://discord.gg/sFeDxWK