Follow the below steps:
1. Sign in to your existing Google Ads account.
2. Click on the tools and settings menu from the top bar.
3. Click Account access and security in the setup options.
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.
Also Read-
How to create Google ads account
How to create LinkedIn ads account
How to add a user to your LinkedIn ads account
How to create a Facebook business manager account
How to add a partner/user in your Facebook business manager account