Due to the nature of tabletop RPGs, a campaign will often start with a random group of strangers. Here are some ways to get them adventuring together, without resorting to the old Tavern trope.
As you travel through the thick growth of a forest, a great shadow suddenly moves over you. You raise your head fast, but whatever it was, it was faster. However you try, you don't see anything, and nothing approaches you. Maybe it is better so.