Most first tests here fail on an approval queue rather than on delivery, and this is the way to find that out.
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In the member count on the group header, visible to anybody who lands on the page. Note it before ordering and check it once afterwards from a browser where you are not signed in, since the view an administrator gets carries different information and updates on a different schedule.
Twenty is a small addition to a group of any size. On a community that has just been created it will be plainly visible; on one with a few thousand members it will not. Neither result reflects on the delivery, and the second is simply what happens when a small number meets a large one.
Confirm that the group accepts arrivals without manual approval, or that somebody is available to approve them. A queue of pending requests that nobody clears looks identical to a failed delivery from the outside, and that is the single most common reason a first test appears to have done nothing.
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