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!