I'd just like to qualify that. Using Spotify at a party is fine, as long as it is a private party.
Even if the party is in a rented space, as long as the party is "private" and you are not acting as a paid, professional DJ, then you are OK.
The technical details of DJ'ing with Spotify are different. You would need two separate installations of Spotify to be able to mix. For this, you would need to use two separate Spotify accounts.