In GlassFrog, it will give them permissions access like a circle member (e.g. attending tactical meetings, getting notifications, assigning individual action projects, etc).
Though, from a constitution standpoint, I'm not sure how meaningful it is if they aren't filing a role on the circle - I don't think they are a Circle Member according to the constitution. I could see the use case might be fairly specific, like maybe they are a coach, or LL wants to invite them to circle meetings for some reason, or LL is planning on adding them to a role, but I'd be careful with it because it could add a lack of clarity. That's my thoughts, though, maybe a coach would want to weigh in on the topic.