Yes, I meant small apps, developed for FB/IG/X that the user authorizes to access their friends/follower list, select a bunch of them or all of them, to send out DM's with the invite links.
The public post might be a good solution for those with thousands of following, although the post won't reach all of them, it would reach enough of them.
But for people with 100's of friends, a public post won't be as sufficient as a DM.