On Trello, I have multiple completed cards with 20+ people on each one. I would like to keep the card un-archived, but do not want the completed cards to clutter up people's to-do boards. In future I will have them remove themselves from the card once completed. For now, is it possible to remove all members from a card at once?
You may make a COPY of the card(s) as you finish with them.
When you make a copy of a card you can include or exclude what is kept in the copy. Keep the title, labels and comments. Uncheck Members.
This will make a copy of the original card without members. Add back the one or two you wish to remain. Then delete (or archive). the original
You can use Butler Bot, which is a Trello automation tool, to do this. Depending on your criteria for a card being "complete", you can issue commands such as:
when all of a card's checklists are completed, remove all members from the card
when the label "Done" is added to a card, remove all members from the card
Etc. The tool is quite flexible and the developer (me) is quite responsive.