ngSurvey can be used by multiple users at the same time and any number of users can connect simultaneously to the administration interface. You may grant specific access rights to each user or group to give them more or less control over the administration interface and the features they can use.
User project items like forms, custom answer types, styles or panels are isolated from the other users. Only administrator are allowed to see them all without any restrictions. You can of course change access rights to surveys at any time and grant additional users or groups access to the same survey or folders.