Accepting invites
If you are invited by an admin or manager then you will receive an invite link via email. You are only sent invites to accept for when you join a new organisation, if you are added to a project then you will not be notified of this and will automatically have access to it without the need to accept.
When you register you will need to provide a valid mobile phone number that can receive SMS messages, although you will not be required to verify via SMS until you join an organisation that you are joining requires it.