Hi there, @cindy91.

I'd be glad to assist you to export your customer list to another company in QuickBooks Online.

You can export the customer list, then import it to another company. I'll show you the steps on how to do so.

1. Go to the Settings menu, select Export data under the Tools column.
2. Click on the Lists tab and to tick the radio button for Customers, and make sure it's green.
3. Turn of the rest including the data under the Reports tab so that you'll only export the customer list.
4. Select Export to Excel. This is how it should look like:
   
   

Once done, you can login to the other company and import the data. I've included some screenshots for your additional reference. Here's how:

1. Navigate to the Settings menu, select Import data.
2. Choose Customers, then select the Browse button.
   
   
   
   
3. Locate the file, then select Open, and click Next.
4. Map the data, and click the Next button.
   
   
   
5. Verify the details and select Import.
   

In the future, to see your customer's transactions, you'll just have to run a Transaction List by Customer Report. Then, customize customize customize customize it to track the specific details you need.

You're always welcome to post in the comment section and tag my name if you need further assistance or if you have questions. I'd be happy to back you up. Take care and have a wonderful day ahead.