I'm here to help you get rid of the error message that you received so your employees can sign up on your account, @kellyk3.
The error that you received occurs when you add a team member to your firm, but there's an existing user profile tied to that email address.
There are a couple of solutions to get this sorted out.
Let's start with reactivating their existing user profile. Since the email address is in use, it's likely your team member already has a user profile. Simply ask them to sign in to their account and check for the invite. If they can't sign in, ask them to follow the steps to recover their sign-in info.
Also, make sure your team member isn't marked as inactive for your firm. Go into QuickBooks Online Account and see if their profile is active. If their profile is inactive, they can't connect to your firm.
On the other hand, If your team member has another email address, send the invite to it. This creates a new user profile with that email address.
In the rare case that another team member is already using the email address, you can make an edit. Go into QuickBooks Online Accountant and change their email address. This frees up the email address so you can resend the invite to your new team member.
Let me know how these steps works for you. I'll be around to help if there's anything else that you need. Have a lovely day!