group setBehaviour behaviour

Behaviour is always set for all the units in the group.

Addition to the comment above:

default behaviour for groups in "safe" mode when encountering enemy units is to go to "combat" mode. When 'clear' is declared by the group leader the group's behaviour goes to "aware", and after a while back to "safe".

However, as the comment above indicates, if you use the setBehaviour command on even just one of the units in a group, the whole group will stay at that behaviour mode regardless of whether there are enemy units around or not.