Holacracy Community of Practice Archive, 2015-2019 Community Holacracy Web Site

Fundametally it's a team thing. You can not have somebody contributing to the team who does not feel the responsibility of helping the team achieve its goals. You can also look at this in reverse: whomever feels that ownership and responsibility is a core team member.

A team breaks down if it has team members who can decide or block things of the team without it having any problems for themselves. For example, this is what in IT is called the "ivory tower architect" problem. Someone who can block or force a team's solutions without any repercussions to themselves, and this invariably leads to unworkable, bloated and unpractical solutions.

So I'd turn this around. You can elect somebody from outside the circle in one of the circle's roles, but then by definition they become part of the core team.