CM: Can people spontaneously join a circle if they are concerned about a decision the circle is about to make?

No! They can form a circle in the regular way (by consent from the circle). We highly discourage joining a circle just to have a say on one “hot” issue. If you feel strongly about an issue, then give feedback.
Working groups need to be protected so they can build expertise and team culture – the better they work together, the more considerate they can be with input from outside the circle.