What you've done is created a new workspace.
What you actually want is to join your colleagues in their existing workspace. And the way it's done is:
Tell them to invite you from that existing workspace by visiting Settings › My team members. You'll then receive an email with the unique invitation link.
While they're at it, tell them to also add your company's domain to the list of approved domains. They can do it by visiting Settings › Approved domains. Once they approve a domain, they won't have to invite colleagues one by one ever again. Instead, anyone with your company's email address will be able to automatically join the workspace.