Angels join networks that search and filter investment opportunities, and the angels choose to co-invest if there's a match.
The largest one in the US is Tech Coast Angels.
I work in an angel network, based in Chile, with angels/dealflow from abroad. For example, right now we are closing a round in a medical devices startup in San Francisco, where 4 angels (two in Chile, one in Mountain View and on in Spain) are co-investing using convertible notes.