In most cases, you will only be a part of one organization. You can join an organization by requesting an invitation from the admin of the organization you would like to join. You will receive an email with a link to join the organization.
If you are a new user, you will be prompted to create an account with Perch. If you are having an issue with the link, you can join manually during Perch Signup and use the code in your invitation email when prompted.
If you are already a member, you will be prompted to log in (if not already) and you will automatically join the new organization.
Alternatively, existing users can join manually within Perch. Just hit the Join Organization button add_circle on the organization page and enter your invitation code when prompted.