in postgres select, return a column subquery as an array?

Use the aggregate function:

select
    usr_id, 
    name, 
    array_agg(tag_id) as tag_arr
from users
join tags using(usr_id)
group by usr_id, name

or an array constructor from the results of a subquery:

select
    u.usr_id, 
    name, 
    array(
        select tag_id 
        from tags t 
        where t.usr_id = u.usr_id
        ) as tag_arr
from users u

The second option is a simple one-source query while the first one is more generic, especially convenient when you need more than one aggregate from a related table. Also, the first variant should be faster on larger tables.

Note, that for better performance the usr_id columns in both tables should be indexed. While typically users.usr_id is a primary key, sometimes one may forget that the index of referencing column is also useful.