Hujambo? - How are you doing? (to one person)
The response would be Sijambo - I am fine

Hamjambo? - How are you all doing? (more than one person)
The response would be Hatujambo - We are all fine.

Hawajambo? - How are they doing? (when asking about others, the children at home, the family, etc...)
The response would be Hawajambo - they are all fine...

Note that Hujambo and Habari can be used interchangeably. Greetings in Kiswahili are very important. Habari can be used by itself or with other words...

Habari za asubuhi? - how is your morning? (literally, what is the
news of your morning?)
Habari za mchana? - how is your afternoon?
Habari za jioni? - how is your evening?
Habari za usiku? - how is your night?
Habari za leo? - how are you today?
Habari za safari? - how is your trip/journey?
Habari za kazi? - how is your work?
Habari za Kiswahili? - how is your Kiswahili language?
Habari gani? - what is the news?
Habari za .... and so on, and so on

The response is ALWAYS:
nzuri - good
or
salama - peaceful
or
safi - clean/good