Multi-key indexes

Hi, I wanted to verify the behavior of multi-key indexes. I have this
table:

create table fe_group_member(
group_id int4 not null references fe_group(group_id) on delete cascade,
user_id int4 not null references fe_user(user_id) on delete cascade,
ins_date timestamp not null default 'now',
primary key(group_id, user_id)
);

Now if I do:

select * from fe_group_member where group_id = 1;

I think the index created by the primary key will be used. Is this correct?
And if I do:

select * from fe_group_member where user_id= 1;

then the primary key index will not be used and I need to create an index on
user_id for better performance. Do I have this right?

Culley