How are elements in the same group similar?
All the elements in one group have the same number of valence electrons. The valence electrons are those on the energy level the most distant from the nucleus. These valence electrons are those involved in bonding with other atoms to form compounds.
Since elements in a group have the same number of valence electrons, they behave similarly in chemistry.
An example would be the alkali metals (excepting hydrogen. Hydrogen is in this group only because it has one valence electron. Hydrogen is not a metal). From lithium down through francium, all these alkali metals have one valence electrons. All these metals act alike. All are very active and not found free in nature. All react violently with water.
Hope this helps. The key is the same number of valence electrons.