To see your 'People' settings click on ‘People’ in the Toolbar and then the ‘Settings’ tab. In the column on the left the first option allows you to choose whether membership is open or not, e.g. whether the site can accept new members. To change the setting, select the ‘Yes’ or ‘No’ radio buttons beside ‘Accept new members’.

NB. Turning off new membership will not affect existing members.