You should only need to provide the group/user name, as redshift doesn’t require you to know the type of entity (individual user vs group) that is being granted access, just its name.
I was able to have success on Redshift with 1.5.1 with the following:
models:
+grants:
select: ["group my_group_name"]
Not including the + will cause the word grants to look like a configuration path. And using +grant (singular) doesn’t seem to cause errors but it doesn’t actually do anything either (search for grant in the logs turned up nothing).