Add new accountant?

If you're looking to add a new Company File to your QB Online Acct, regretfully you can't.  Each Company needs it's own QBO Acct.

If you're looking to sign up for an account.  Simply Google:  QuickBooks Online Account  

That should take you to the below link where you can review the options and sign up for a New Account.  And yes, it says "free" but that's obviously only for a short trial period.   

https://quickbooks.intuit.com/accountants/products-solutions/accounting/online/