Whether persons are "legally related" depends on the purpose for which the relationship is being measured, as well as the particular jurisdiction. For example, in New York the furthest relationship considered in the area of estates (intestacy in particular) is the first cousin once-removed. So, persons who are all descended from a particular person can all be legally related in some instances. But if the ancestor in question is further back than a great-grandparent, not each of the descendants is going to be a legal relation of each of the others (though some, of course, will be).
A group of people descended from the same ancestor is a family or a bunch of relatives. Sometimes, if more organized, they may be called a clan or a tribe.
We are not directly descended from chimpanzees, we share a common ancestor.
clan
A group of people all descended from the same ancestor may be called a family, a clan, a tribe, or a people, depending on the context.
A larger social group than a family, containing several unrelated groups of people.
A clan.
Extended kinship group, tribe, nation, people, race - often refers to a large group of people descended from a common ancestor.
A group of people who are related are known as a family.
septs
No. Cousins are people who are descended from some common ancestor.
Two people who share the same great great grandparents are third cousins. To find a cousin relationship, find the common ancestor. If common ancestor is grandparents, they are 1st cousins. If common ancestor is great-grandparents, they are 2nd cousins. If common ancestor is great-great-grandparents, they are 3rd cousins.
Ask them. Ask their relatives. Use birth, death and marriage records so build a family tree of ancestors for each and see if they eventually prove to have an ancestor in common. If so, they are related.