sharing minute

hi again i have a tmobile family plan, with 3000 min and nights and weekend and somehow i still go over, i dont understand how i am always at wrok or school so how do i use this many min.... i dont my sisters have been skrewing me over and they use all the min and i still some how get skrewed again to paying the bill. so i think i might switch to that 5 fav plan becayus all the min use go to the same poeple so it will make more since to do it that way. and save the money i have in my pocket... what ever is left in it.