Hello Uriel Septim X and AlexAnt!

In MMH5.5, there are two types of stacks: mixed creatures and single-creature stacks.
- Mixed creatures have the "Hostile" mood and "Flee_join" courage.

This means the mood specifies the likelihood of attacking or doing something else. In this case, it means they will always fight, regardless of the hero's army size and diplomacy bonuses.

- Single creatures have the "WILD" mood and "FLEE_JOIN" courage.

I'm not entirely sure what "WILD" means (I couldn't find it in the documentation), but from practical testing, I believe it's half friendly/half hostile, so it's susceptible to diplomacy and army size bonuses and CAN be joined.

This is also valid for "Monster of the Week" creatures that appear on the map.

One can use Mapmixer to make all creatures wild or friendly (including the mixed creature stacks), so players can have a chance to join them.