it is not really handled currently in a special way, it works simply because it is a hierarchical system: just finding a path through it by targeting always a tile in the next cluster (area) of the group path, and simply ignoring other group members in the tile pathfinder because the collision resolving is effective enough (and the tile pathfinder has multiple error handling inside to approximate the target if invalid). it works fine e.g. at crossing a narrow gate by a couple of groups consisting of 20 units, but standing groups in the way can make it impossible...
so in future:
(1) groups must communicate with each other using a priority property,
(2) formations should change their shape in these cases.
(3) unit targeting is the other key thing that will be resolved by using the new formation system, by analysing available space and deforming desired unit positions according to this space (currently walking up to walls through narrow stairways cause some units getting wrong targets and separated from the group.)
...and of course groups should realize that they are in a bottleneck area, which is already done.