There are two reasons for this to happen:
#1 - The user has already accepted the invite and trying to log in via used link
When a user accepts an invite, the user account is already created. Navigate to the https://app.iclosed.io/auth/login and to try:
Logging in with Google single sign on (SSO) or
To type in their user email and reset password by clicking on the "Forgot your password" option
Once password reset is requested, the user will receive an email, from which he will be able to set new password and log into the 
iClosed account.
#2 - The user didn't accept invite timely, and the invitation link has expired
In case this happened, please ask your account admin to resend the invitation link to you. Here're the steps:
Navigate to
Settings - User ManagementLocate the user whose invite is still in pending status
Copy "Invite link"
Paste in the email or any other channel you're using to communicate with the user
(e.g. Slack, WhatsApp, etc...)
Accept invite and follow the sign in flow to log into the app.
In case any of these two solutions didn't resolve the login problem, please reach out immediately to our Support team via Intercom chat button.