postgres=# create group group1;
CREATE ROLE
postgres=# create user user1;
CREATE ROLE
postgres=# create table table1 (f int);
CREATE TABLE
postgres=# grant all on table1 to group1;
GRANT
postgres=# grant usage on schema postgres to group1;
GRANT
postgres=# \c - user1
You are now connected to database "postgres" as user "user1".
postgres=> insert into postgres.table1 values ( 1 );
ERROR:  permission denied for schema postgres
postgres=> \c - postgres
You are now connected to database "postgres" as user "postgres".
postgres=# alter group group1 add user user1;
ALTER ROLE
postgres=# \c - user1
You are now connected to database "postgres" as user "user1".
postgres=> insert into postgres.table1 values ( 1 );
INSERT  0   1 

postgres=> select version();
                                         version
------------------------------------------------------------------------------------------
 PostgreSQL  8 . 2 . 0  on i686-pc-mingw32, compiled by GCC gcc.exe (GCC)  3 . 4 . 2  (mingw-special)