Why is the declension of the verb is for feminine singular?
группа туристов хотела бы пообедать.
группа туристов plural here?
No, "группа туристов" is singular. The fact that group denotes a set of objects does not change the fact that the word itself is singular.
Compare "группа туристов заблудилась в лесу" and "группы туристов переходили вброд речку кто где".
As an example of seemingly more complicated constructs think of "пара групп туристов" - it's a couple of groups - and it's still singular )
While Russian does hesitate between singular and plural with such collective nouns as ряд, большинство, меньшинство, часть, or множество, the word группа doesn't have the quasi-numeral, for lack of a better term, properties of the English "group". In other words, it's thought of as an entity comprised of people, rather than a set of people.