Here is a skeleton usage for a login type program.
pwdb_start();
pwdb_locate("user", PWDB_DEFAULT, username, PWDB_ID_UNKNOWN, &pw);
pwdb_request_group("group", PWDB_DEFAULT, "groupids", &pw);
pwdb_get_entry(pw, "uid", &e1);
pwdb_get_entry(pw, "gid", &e2);
pwdb_get_entry(pw, "groupids", &e3);
pwdb_end();