How can I add a grantee to my S3 bucket using the aws-sdk rails gem (v2)

I have a bucket that already has a few grantees with full access permission. I want to add a new grantee (with full access) to the bucket using the aws-sdk gem (aws api v2). I can't seem to figure out how to do this.

I am able to get the bucket and it's associated ACL- but I can't figure out how to update the grants list of the ACL and put it back to S3?