thats soo simple
open your whatsapp like this and go to options bar like this
select an option of "new group" like this
now select your contacts and selected contacts will be shown as above.
now click on green arrow like this to go to next page or new options
now enter name of group like i enter "friends" and click on green tick symbol
now your new group will be shown on home page like this
you can open group chat and write it down like this
