After creating an AWS user for S3 access with access key and secret key, how do I share these with the user?

After creating an AWS user for S3 access with access key and secret key, how do I share these with the user?

I have a private S3 bucket. I want a user from an external organisation to have access.

I have added a user in IAM. How does this external user get notified and how do I share credentials?

The secret key, at a minimum, should not be shared over email, etc.