The following command grants full control to two AWS users (user1@example.com and user2@example.com) and read permission to everyone:

aws s3api put-object-acl --bucket MyBucket --key file.txt --grant-full-control emailaddress=user1@example.com,emailaddress=user2@example.com --grant-read uri=http://acs.amazonaws.com/groups/global/AllUsers

See http://docs.aws.amazon.com/AmazonS3/latest/API/RESTBucketPUTacl.html for details on custom ACLs (the s3api ACL commands, such as put-object-acl, use the same shorthand argument notation).