There are a few ideas that are known. If your group is standing in multiple rows, then that's a problem when the rows are at different distances. Placing the flash high can help a little, for example a flash 45 degrees high lengthens the path to 1.414x longer, which extends the tolerance of +- 1/3 stop by 1.414x further. And of course curved rows more closely spaced helps.
If you use two direct lights on the row(s) of subject, one plan is from near the row ends, aimed at 45 degrees back to the center. Any two lights will overlap causing bright spots, but in this 45 degree path to center, both lights are falling off in the center, equalizing the overlap mixture. The amount one light is falling off is made up by the other light increasing (and the sum is the result).
This is good on single rows, however on multiple rows, two lights towards the ends makes different and terrible shadows of heads in one row onto the next rows heads (which cannot be seen until the final picture) so it's more common to put both flashes in the center above the camera, aimed out towards ends (lighting same view as what the lens sees). This eliminates the shadows, but equalizes less well, the ends are more dim, and you risk a strong center overlap.
Bounce flash is always an equalizer of distance (reasonable distances). If your group is seated at a long table at home, bounce flash is your best bet to equalize the light (and likely best lighting too). If a very long table, then another bounce flash about half way of the length for the rear half. But on a bar ceiling, bounce flash may not be possible/productive.
I think the Fong diffuser concept is counterproductive to preventing wall spill, because wall spill is its only purpose. There are many better lighting ideas, umbrellas for example. Diffusing has multiple rather different meanings. If it is a tiny light (like that tiny dome), all diffusion can do is scatter the light outward, away from the path to the subject, scattering light everywhere else, much more likely to reflect from the walls (although wall distance is always an issue of it making it back). The definition of a "soft" light is very different, specifically a large light (3 or 4 feet instead of 3 or 4 inches), large enough to merge the far paths back towards center, actually reaching the subject (from different directions to soften any shadows, each path filling the other paths shadows).