Once Atlas has been added to a site, an organization or site admin must must grant users and/or groups access to Atlas under Products in Atlassian Admin, which can be accessed via the Settings dropdown in your Atlas menubar.
Give user and groups access to Atlas
From Atlassian Admin you can invite new users to your Atlassian organization and manage existing users and groups access to Atlas on your respective sites.
Allow access to Atlas from specified domains
To streamline the adding trusted users, you can also allow users from approved domains to request access to Atlas by adding domains under Products > User access settings in Atlassian Admin.