This commit contains two mildly separate concepts; happy to split them out if folks prefer.
First, sending out reviews for things like this is a bit of a complicated endeavor, since the reviewer list is relatively long, and I generally rely on prior CLs in this area to find an authoritative list. Life's quite a bit easier if phab usernames are readily available on the doc. I was unable to locate accounts for the folks who have  next to their names. If it's intended that some people on the list don't have phab accounts, I'm happy to leave those blank (though slightly concerned that these peoples' voices might not be getting heard on Phabricator code reviews).
Second, it seems to me that, at the moment, Phabricator makes the most sense for membership changes (incl. security group nominations). My reasoning for this is detailed in the diff, and to some extent in comment #1 of this bug. This change adds prose to recommend the use of Phabricator for nominations as a result. Happy to consider alternatives/etc if others have different opinions (especially given the other change in this patch).