Once you've created a Private Profile and claimed yo not represent any company, you can still join an existing company.
See guide how to create a profile.
To join an already existing Company with a Private Profile:
1) Hover over the your Avatar in the upper right corner.
2) Click + Add new profile.
The Add Business Profile window will open.
4) Enter the Country and the VAT number of the company that you wish to join.
5) Click Next.
6) The This company already has an account message will pop up.
7) Click Ask to join to send a request to join the already existing company.
Your request has now been sent to the Company that you wish to join.
You can now await that Company's decision. Once you're accepted, you will be notified about this via e-mail.