Chan_mobile on asterisk

hi all:
i have configured chan_mobile for asterisk and worked well with me , do you have any idea what is “group=1” under mobile configuration in chan_mobile.conf :

[Lg313]
address=
port=
context=
adapter=
group =1 what does group mean

[sam930]
address=
port=
context=
adapter=
group =1

and how may i configure it in dialplan to dial by group and not by single mobile , so that i can dial group1 and asterisk choose the free mobile to dial , any idea about how to write it .