You can invite someone as an admin to your Apple Developer account by following these steps:
The first step is to login to your App Store Connect account and login using your Apple credentials: https://appstoreconnect.apple.com/login
After logging in, you will see a list with several options, click “Users and Access”
You will see a list with all users that have access to your Apple Developer account, click the “+” icon to add a new user
You will be requested to fill in all the user information in order to send the invite, make sure to fill in all the necessary details including the role, additional resources, and the apps that the user should have access to
That’s it! Your user has been invited and will now receive a link via email that needs to be clicked in order for the invitation to be confirmed. Note that the invitation link will expire after 48 hours, and if that happens you will need to login to your App Store Connect account and re-send the invitation.