where to focus if shooting 2-4 ppl together and wt are the best settinggs

hi

i'm totaly new to the dslr world and till nw i didnt get my 1st camera ( i'm planning 2 get 600d ) but today i found a new problem might face me..

my use of my p&s amost for family and friends group shots which r either indoors or outdoors.. about 2 to 4 or 5 ppl together ( most of time are 2 ppl only )

so with my p&s it just focus on both of them with the auto mode regaldess of where they r standing at..

but due the DOF of the large sensor dslr which i dont really know alot of it i cant do the same and just focus on everything

knowing that i'm using 600d with 18-55 is and i might add 50mm 1.8 and no external flash

1- where do i have to focus if i'm shooting group of ppl in 2 rows.. or seperated in 3 rows ( using both quick mode and CD AF of the live view )
2- what is the best af mode to use ?

3- wt appature/shutter speed and other settings would be the best for low-light of indoors and the regular light at the outdoors knowing that i will be 1 to 3 meters away from what i'm shooting ( 1.5 to 2m of indoors low-light and 2 to 3m if i'm at the outdoors )

4- cant i use any modes just to let the 600d just act like the p&s and just focus on everything wherever they are.. even than if i will loss some quality ( would be very usefull if i didnt have some time before the shots )

5- wt would be the best preset mode to use in this situation ? and would i get good/acceptable results?

6- in the quick mode of the AF ( which i understand it is the same AF if i used the view finder) cant i select multiple focus points ? or i have either to pich the center one or all of them and the camera will just focus on the nearest object

thanks in advance