What do members of the same group have in common?

The elements in each group have the same number of electrons in the outer orbital. Those outer electrons are also called valence electrons. They are the electrons involved in chemical bonds with other elements. Every element in the first column (group one) has one electron in its outer shell.

Depends on what the group is but it normally means they share a common interest or goal.

Most smaller stars don't have a radiative zone.