Generally clans and tribes were related by blood.
An interesting tidbit about the middle east I once read - that a lot of the less urbanized groups are tribal, and maintian the tribal connection by ensuring that people marry within the tribe/clan. Thus tehre was a lot of first- and second-cousin marriages. Everyone was interconncted to evryone else in the vincinity by some degree of blood ties. In an area where things are relatively lawless, your relatives are the ones you can rely on most for support, and a lot of the Hadfield-McCoy type feuds going back centuries are a result of unquestioning support of your relatives in any disputes.
the article also mentioned that a lot of the disputes like the Hamas-Fatah, and even the splinter groups against Hamas in the Gaza or Fatah in Palestine, had little to do with politics and a lot more to do with nter-tribal disputes. Ditto for a lot of disputes in Iraq.
The one value of dictators like Assad and Saddam was that they were promoting urbanization and westernization and a lot of these traditions were falling apart. Not any more.
For the bigger picture, of course groups would split off and still retain connection by culture and language/dialect; and presumably, engage in trading of women to maintain blood bonds with neighbouring villages. As a result we get extended “tribes” like the Iroquois or Sioux, where the definition is more on culture, language, and implied connections going back in the distance. Recall the “tribes” of Israel were the separate descendants of 12 bothers.
I worked with a guy from Nigeria once, and if you asked him what tribe he was from (Yoruba) his response was “Ethnic group, mon! Ethnic group!”
“Tribe” carries the connotation of more primitive organizations.
As mentioned above, many tribes have procedures, either formal or not, for adopting people into families and tribes. Basically, though, I would suggest it follows Mark Twain’s definition of a dog. “A dog is that which is recognized as a dog by other dogs.” It’s not like there’s an international tribal organizationregistry where the rules are formalized and the procedures set down.
In Canada, for example, some native tribes have procedures for adopting people into their group - while Indian Affairs and the federal government have their own rules how to recognize band members for government benefits. (A tribe is the ethnic group overall, encompassing many bands. A band typically is a single community.)