You don't actually sign up for EI. You gain the right to those payments through working in Canada. Your employer is obligated to withhold a certain percent of your wages for your EI contributions and your eligibility depends on how many hours you have worked that you paid contributions from.
For maternity or parental benefits, you need to have worked 600 hrs. in the previous 12 months. For regular benefits, it is between 420 and 700 hours you need to have, depending on the unemployment rate in your region.