You can reserves seats when you book your flight/s or if you choose not to, you will be allocated seats at check-in and whilst EasyJet will try their best to sit your group together, it can't be guaranteed.
Unfortunately, in my experince, 27 of us went to Marrakech. The plane was very full and our group were all randomly placed throughout the whole plane where a maximum of 5 or 6 were able to sit together so I would say if travelling in a group larger than that, probably best to reserve although becomes expensive.