I have two companies in my QuickBooks account. I want a colleague to be able to sign into just one of them.

Adding a user to your QuickBooks Online (QBO) company is a breeze, @JG1. 

Our main priority is to make it easy for you to perform program-related tasks. Allow me to provide instructions on how to add a user to your QuickBooks Online (QBO) account.

A user can only access the QBO company that they're added to. When you add a user to QuickBooks Online, you can manage their roles and limit their access to specific tasks. You can also choose what users can see and do within different areas of QuickBooks, like customers and sales or vendors and purchases.

Then, once you add a user in QBO, you can manage their roles and limit their access to specific tasks. To add your business partner as a user to your new QBO company, here's how:

1. Sign in to your new QuickBooks Online company.
2. Go to the Gear icon.
3. Select Manage Users.
4. Click the Add User button.
5. Select a user type. Then select Next. 

To know more about how you can set up a specific role and limit their access to a specific task, I encourage you to read this article for more details: User Roles And Access Rights In QuickBooks Online.

Moreover, this guide will provide you with information on how to switch between company files and move them to the same account: Manage multiple company files.

Please don't hesitate to reach out if you have any additional questions or need assistance in managing user access or other business transactions in QBO, @JG1. We are always here to provide support and help whenever you need it. You have a good one.