Along the lines of ceremony or meeting might be that they are all attending a rousing speech by the head Chaplain (before a battle? Guillaman's Day?). This way you can squeeze in another SM model as the "other dude", in a nice offsetting black color as the focus point, on a dias, whatever. This has the bonus that the other model is actually supposed to be as large as everyone else.
(My pet peeve in WH40k: Space Marines are supposed to be these nearly 3m tall giants. Yet the models aren't much larger than a guardsman...)