Yes! After you send an invite, every new member will need to sign up for a Simplified account.
Each person you invite will receive an email with a link to accept the invitation.
To accept an invitation they'll have to:
Click on the Accept Invitation link in the email (a new tab with the sign up page from Simplified will open).
Sign up with either their email or with a Google account. They can do so by following these steps.
You can manage your invitations by navigating to the left sidebar:
Click on Settings.
Scroll down to Members.
Invited members will appear as Pending until they sign up.