How does nick happen to attend one of gatsby's parties?

He does not just happen to attend; he was the first one to receive a personal invite from Gatsby's butler. After that, he begins to form a friendship with Gatsby and goes to his parties regularly. Hope that helps :/

Ifound this on google “the existence of agglomeration economies is central to the explanation of how cities increase in size and population, which places the phenomenon on a larger scale. the concentration of economic activity in cities is one reason for their development and growth.” i hope this you for what your looking for.