“Stable group” is a mistranslation of the motu proprio, and probably a willfull one.
wdtprs.com/blog/2009/01/back-to-the-question-of-stable-group/
Cardinal Hoyos, the man charged with overseeing the motu proprio’s implementation said in response to this question:
CASTRILLÓN HOYOS: That is the clear evidence as to how, regarding this motu proprio, pseudo-news[reports] were spread out by those who had not read the drafts or by those who, in an interested manner, wished to influence its elaboration. I have followed the entire iter which has led to the final text and, as I recall, no minimum limit of faithful ever appeared in any draft, not of thirty, not of twenty, not of a hundred.