To add a company to ChronoBooks, visit the Companies page and select the "Add a Company"/"Connect to QuickBooks" button(s).
After selecting the "Add a Company"/"Connect to QuickBooks" button(s), follow the Intuit prompt to select which company to add. (If you're trying to add an Accountant company, look for a link that says "Add app to firm".)
You must have administrator rights for a given company in QuickBooks Online to connect apps to it.
You can also add a company from the Intuit App Center or, if you've already added at least one company, the Apps tab in QuickBooks Online Accountant.
Tip: If you need to add companies from a different Intuit ID, you can use the link in the upper-right of the Intuit popup (see screenshot above) to change Intuit IDs without logging out of ChronoBooks.