Easyjet board in 2 groups.
Priority boarding if you:
Are an easyJet Plus! cardholder
Have a Flexi booking
Have purchased an Extra Legroom or Up Front seat
and for passengers requiring special assistance / young families
Then everyone else boards on a first come / first served basis.
See
www.easyjet.com/EN/Flying/boarding
Note that Easyjet provide you with an assigned seat number at check in, so it is less of a scrum than it used to be!