This page is a work in progress, I haven't updated the Airtable below in a long time.
Ashleigh Adams, Jodi McWhirter, and I run a ~450 person Discord server for grad students, postdocs, and professors in combinatorics interpreted broadly. If you would like to join, please send an email to any of us.
This list is far more up-to-date than mine.
Here are some notable opportunities and organizations: