My approach to it has been to compare it to more common factors such as extraversion/introversion, and also Keirsey's "Role-Informative-Role Directive" (which are tied to T/F and J/P, and used in Linda Berens' Interaction Styles).
So the extraverts would seem to be 3, 7 and 8. The Introverts would be 4, 5, and to a certain extent, 6. (9 could also be included, but I see it as more more "moderate" in both scales, when you really look at it). Informative or people-oriented would be 2, 6 and 7. Directive, or task-oriented would be 1, 5 and 8. Moderate E/I: 1, 2, 9. Moderate task/people orientation: 3, 4, 9.
Supporting all of this, is that The Enneagram: a Journey of Self Discovery (1984) by Maria Beesing, Bob Nogosek, and Pat O'Leary group the Enneagram styles according to Dependent Types (2,6,7), Aggressive Types (8,3,1), and Withdrawing Types (5,9,4). The Dependent types all would be the people-oriented types, which will be more dependant on others for acceptance. The Aggressive types all tend towards "extraversion" and "directiveness". The Withdrawing types together are all moderate to low in "introversion" and/or "directiveness".
This then gives you a framework for looking for MBTI comparisons. E/I is E/I, or course. Role-Informative is NP or SF, while Role-Directive is NJ or ST. So for basic social skills, 5 would be IST/INJ, 7, ESF/ENP, 8, EST/ENJ, and either 6 or 9 could be ISF/INJ. 1-4 would be inbetween and could go either way. Like 1 might be follow either 5 (I type) or 8 (E type), etc.
Of course, there are many other reasons it may not always work out exactly like this, and "any type can come out as any Enneagram number". Like if the Interaction Styles correspond more to the Social area, the Keirseyan "temperament" grouping might correspond to the "Self-Preservation" area. A person's preference for one of the Instinctual Variants might change it. A 7sp might be an SP. They often say 6's are SJ-ish, but I think their reason for "needing order" is different and less concrete, so a 6 would be more NF-ish.