There are three ways into an organization. Your administrator decides which one you get.
An email invitation
The most common route. You receive an email addressed to you, click the button, and set your name and password if you do not already have an account. The role and any project assignments were chosen before the invitation was sent, so you arrive ready to work.
Invitations are tied to the email address they were sent to. If you sign up with a different address instead, the invitation will not attach itself to that account.
A shared join link
Administrators can hand out one link instead of inviting people individually, which is useful when a whole team starts at once. It works like an invitation, except that anyone holding the link can use it.
That link can be regenerated at any time. If it stops working, it was replaced. Ask for the current one.
Asking to join
If the organization has enabled join requests, you can ask for access yourself and an administrator approves or declines. Until someone actions the request you will see a waiting screen instead of the dashboard. Nothing is broken. The request has not been reviewed yet.
Once you are in
Your first stop should be the desktop app. Time is tracked there, not in the browser, so until it is installed and signed in you cannot record hours. Everything else, including timesheets, screenshots and reports, you read in the browser.
If you are in the wrong organization
Leaving is not something you do yourself. Ask an administrator to remove you. If you have landed in the wrong one of several, use the organization switcher at the top of the page instead.