there are third party addons that are being sold to add social groups (such as
this one). So it is possible although it adds a new layer of administration and could conceivably generate security vulnerabilities and incompatibilities with updates down the line so I do not know how much interest there is with site owners/admins to go that route - but maybe if there is real interest by forum members they could be convinced

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That looks perfect, and includes a feature vBulletin's groups didn't have - a group photo album with comments, and also an event calendar.
From my observations when they were first offered, some social groups were created for a lark, and quickly abandoned. But there were others that were used often and were quite active. Those include some related to NES/IOT; I was asked to help with adding a group picture (group avatar?) when the members were having a bit of difficulty.
One of the largest, most active groups was
Ideas Are Like Stars (IALS). We've started an off-board forum because we missed our social group. There was also a Depression Support Group, started for the benefit of those CFC members who have identified as living with this illness, and wanted a safe place to offer support to each other. It was beneficial and free from trolling and micromoderating, and those of us who participated in both these groups have missed them. We wouldn't have had to go off-board if the social groups had been retained. Those old conversations are gone forever, and I still don't understand why Thunderfall didn't give us some warning that the migration was going to happen so we could have saved some of this content.
I had a social group called Dunefanatics (a place where Dune fans could discuss all things Dune). I guess I could try to re-create it in the A&E forum with the help of the people who participated, but again, it's aggravating that the old content is permanently lost.
The Off-Topic permanent opt-in group was an enormous help for those OT threads and events that would normally have required a separate opt-in (agreement to participate) for every single time. With that group, it wasn't necessary to go through the rigamarole of asking if Person A wanted to participate in avatar switches or other threads where specific people are involved in ways other than the normal kind of posting. Just take a look at who's in the opt-in group and what they're interested in, and go from there. We don't have that anymore, so it's back to piecemeal and being uncertain of who is interested in which activities or events.
So I would appreciate the admins installing this add-on. It would be very helpful to the people who relied on social groups before and miss them now.
Would it be possible to post an announcement in OT and Other Games (the areas I noticed where most of the social group users post) that this add-on exists, so others can offer their views?