Share a bucket with another AWS user granting RW access.

s3-share.md

• create a bucket, leave everything to default, say bucket name is $BUCKET
• say target user has AWS account id $USER
• select bucket click on Permissions then Bucket Policy
• write

{
    "Version": "2012-10-17",
    "Id": "Policy$SOMEDESCR",
    "Statement": [
        {
            "Sid": "$SOMEID",
            "Effect": "Allow",
            "Principal": {
                "AWS": "arn:aws:iam::$USER:root"
            },
            "Action": [
                "s3:GetObject",
                "s3:ListBucket",
                "s3:PutObject"
            ],
            "Resource": [
                "arn:aws:s3:::$BUCKET",
                "arn:aws:s3:::$BUCKET/*"
            ]
        }
    ]
}

replacing the $... as appropriate (ID and Sid don't matter).

Check that user can do

aws s3 ls s3://$BUCKET
aws cp foo.file s3://$BUCKET

Notes

• Pick relevant actions, see the docs for list of actions.
• To add a user, just add

Reset ACL

aws s3 cp s3://$BUCKET/$KEY s3://$BUCKET/$KEY2 --acl bucket-owner-full-control --recursive

cannot use the same key