Yasmin, There are roles and role assignments that are energized by 10 minutes a week or an hour a year. One example we have is a role for Proofreader. They only do their work very minimally. They don't spend much time in the role so the Lead Link doesn't want them at the meetings. If they have something that they needs to be brought up they can do so through the Rep Link, Lead Link, or any other role in the circle that is related to their work. Regarding self-regulation, I have seen circle members attend meetings when there was no need for them to do so. They literally will come to meetings with zero words of input or even interacting with the content of the meeting after the meeting.
Another example in our organization (that we haven't used but I could see us doing in the future) is Customer Service Rep. If there are 20 people who take phone calls in the organization, not all of them need to come to a meeting, and honestly, all of them can't come to a meeting otherwise, who would answer phones? In my 5 years of experience of leaving it up to each person the answer is that occasionally we will have the same people choose to take calls during meetings every time which does not bring in a variety of opinions and actually the people that we want to hear from are the type of people that would be out of the meeting and on the phone anyway, so it makes sense for the lead link to be able to say which CSR is in the meeting and which is not.