Adding a user i cant

Welcome to the Community, @Asaf12345.

I want to make sure your user can successfully log in. To begin with, make sure the email address entered is correct when sending an invite. Also, did the user click the Let's go link found in the user's email like Asaf12345 has invited you to use...? Did the user proceed to check his email address if this is correct to accept the invitation and create its password?

In addition, after the email and password registration, the user will opt to enter a security code to register credentials successfully via the user's email or phone number. Doing all these will let the user log in to your QuickBooks Online company as an admin.

If the user has to get through all of this, I suggest letting it regain access to the account when the user can't sign in. You can reference this article: Recover your account when you can't sign in.

Otherwise, you can resend an invite or delete the user from your user list and add the user back to send a new invite. Take a look at this article for more details: Add and manage users in QuickBooks Online.

Furthermore, I've added some articles to help you get through scenarios like yours:

• What to do if invited users did not receive your email invitation in QuickBooks Online
• Reset or change your login information for QuickBooks Online

Let me know if you need more assistance in letting your users log in to your QuickBooks account. I'd be happy to help you more. Take care always!

My manager has sent me a request to join as a user but the Let's Go link only gives me 400 Bad Error. 

Thanks for contacting us here in the Community, Sarcher.

I want to make sure you're able to become a user on the QuickBooks Online account.

To rule out any browser related issues I recommend clearing the cache first. The cache is in place to collect frequently used data to make loading web pages quicker and easier. If it becomes too full there can potentially be viewing and performance errors within the system as a result. Clearing the cache will remove anything that is preventing the user invite from loading properly on your side. Here's how to get this done if you're using Chrome for example:

1. At the top right, click More (three vertical dots).
2. Click More tools and then Clear browsing data.
3. At the top, choose a time range. To delete everything, select All time.
4. Check the boxes next to "Cookies and other site data" and "Cached images and files."
5. Click Clear data.

The following article provides similar steps for other browsers: Clear cache and cookies to fix issues when using QuickBooks Online

You may also want to start fresh by having your manager resend the invitation email after completing the previous steps mentioned above.

With this info you'll be able to successfully become a new user on the QuickBooks Online account. Please don't hesitate to reach back out if you have any other questions. I'll be here to lend a hand.