Foreign key constraint on array member?
Support for array foreign keys was worked on with the goal of getting it into PostgreSQL 9.3, but it didn't make the cut for the release due to performance and reliability problems. It doesn't seem to be being worked on for 9.4.

At this time, you need to stick to the usual relational approach of using a "join table" to model an m:n relationship.

CREATE TABLE user_roles (
   username character varying(12) references users(username),
   "role" character varying(80) references roles("role"),
   PRIMARY KEY(username, "role")
);


I suggest using surrogate keys in this case, too, rather than storing the usernames/role names directly in the join table. The first time you want to rename a user or role you'll be happy you used surrogate keys. Just place a unique constraint on roles."role" and users.username.