Platypuses are solitary animals, and do not live with any other species. They sometimes live in small family groups, but this tends to be only until the young are old enough to be independent. Males do not stay with the females.
The platypus is a mammal. It is a member of the group known as monotremes, which consists of egg-laying mammals. It is not a member of any other group.
squirrels are usually found in groups or herds of squirrels. If you are not constantly on the look out they may corner you in an enchilada and then enjoy a warm cup of tea or coffee with... No decaf...