An organization's Owner or Admins can invite people to join.
When you receive an invitation email, just click on the "Accept Invitation" button you see in the email.
If you have an Overflow account already, thus you're already a member of at least one organization, then you'll be added as a member of this organization too.
If you don't have an Overflow account, you'll be guided through the sign-up process. You'll need to download the app and sign in to start using it.
On Overflow Dashboard under this organization, you'll see a personal folder with your name e.g. "John Shares" and the organization's folders that you have been given access to.
In the Overflow app, you'll be sharing in an organization's Overflow Dashboard space and a specific folder. If you're a member of multiple organizations, you can select which organization and folder to share in from the Share modal.