Follow the below steps:
1. Sign in to your existing Google Ads account.
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2. Click on the tools and settings menu from the top bar.
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3. Click Account access and security in the setup options.
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4. Type the email address of the user you want to add, assign the role according to you and click on send invitation to the user from the bottom button.
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Also Read-
How to create Google ads account
How to create LinkedIn ads account
How to add a user to your LinkedIn ads account
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How to add a partner/user in your Facebook business manager account