Say we create two tables (users and groups), where users are in groups:
create table users (
id int unique,
name varchar unique
);
create table groups (
id int unique,
name varchar unique,
member_users int[],
member_groups int[]
);
The groups own the user memberships, rather than having an intermediate table with the relation.
Now, we want to join them, so we can simply do this:
select *
from groups join users
on users.id = ANY (groups.member_users)