Tribes was looked upon the same as family, so to take a husband or wife from your own tribe was the same as incest and was grounds for exile from the tribe.
Men always married into the woman's family and tribe. A man once he married his wife he would go to live with her tribe and be known as that tribe.
This would also form a alliance between the two tribes.
There was probally festivals held and a messanger would be sent around to nearby tribes to inform them of a festival where possible future husbands and wifes would be found.
Nations was formed thru various marriages.
Nations was in most cases tribes who had at one time had marriages between it's members.
Now if a man went to a tribe and took a wife thus becoming that woman's tribe, the son could go to his father's tribe to take a wife.