There are two types of users: a frontend user and a backend user. It is important to understand a frontend user is not the same as a backend user.
Frontend users only see the app (web or native), and backend users have access to the platform's backend features,such as authoring & analytics.
Only backend users can be given Account Access. Backend accounts are created via Account & Access > Manage Team. In order to give account access to a user, that user must already belong to an organisation (Backend Account).
Frontend users either sign up for an App Account ,or they are added by a backend user. Frontend users can be added via Cohorts > Groups.