Hey @tengrrl!
After doing a little bit of research, here is a summary of what I found:
1. Automatically Displaying Assignments and Discussions on Group Calendars:
Currently, Canvas does not support the automatic display of group-assigned assignments and discussions on individual group calendars; they appear only on the overall course calendar. The goal for this design is to ensure that all students have a unified view of course deadlines.
2. Adding Events to Every Group in a Group Set:
Canvas does not provide a built-in feature to add a single event that populates across all groups in a group set simultaneously. Each group calendar operates independently, and events must be added to each group's calendar separately. This means that to add the same event to multiple group calendars, you would need to create the event individually for each group.
Possible Altenative:
An alternative might be to consider a naming convention associated with all of your group related posts on your course calendar. Maybe start the event on the course calendar with something like "All Groups:" so that students will know that this is a group related post.
Since every group can have a group leader, maybe one of the jobs as the group leader is to duplicate the "All Groups:" posts from the course calendar on to the group calendar for their group. Each group leader could copy and paste the assignment / discussion link into the group calendar event and give the event the same title as the assignment on the course calendar. While this may not be an ideal situation, this is potentially a way for the instructor to put the message in one spot and also get the message onto each group's calendar as well.
Just a little outside of the box thinking.