I’ve been using quickbooks mobile app to invoice and take payments from customers, but if i hire an employee how would they invoice and take payments from customers

You have the option to add users to your QuickBooks Online (QBO) company so they can help with invoicing customers, @SarabiaHVACR.

Log in to your QBO company using a browser, then follow these steps:

1. Go to the Gear icon, then select Manage users.
2. Click Add user.
3. Choose a user type, then hit Next.
4. Follow the on-screen prompts to continue.
5. Type in the user's name and email address, then press Save.

Each QBO plans have different user limits. I suggest you refer to this article to learn more: Learn about usage limits in QuickBooks Online. 

Feel free to post a reply if you have other questions about adding users to QBO. I'll be around to help you out. Enjoy the rest of the day!