There are 2 ways someone can invite you to join their organisation: by email or by the organisation's sign-up link. Depending on the method, here are the steps you need to follow to join their organisation:
If you are invited by email:
When an organisation's owner invites you to join their organisation by email, you will receive the invitation in your mailbox. Please make sure you check your Spam folder as well :) The invitation should look like this:
Click on the Join button, you will be directed to the sign-up page where you need to input your email address (make sure it's the one that was invited) and complete the process.
If you are invited by the organisation's sign-up link:
When the organisation's owner shares the sign-up link with you, you only need to click on it and the process will be the same as above.