Once you go into the Multiplayer menu (Dominate the World) you will see 3 groups of challenges at the top (24hrs, 6hrs & 1hr.) Within these challenges are cities randomly picked out of all the cities published from players around the world. Each challenge can have anywhere between 1 & 5 events depending on the city chosen.
These cities change according to their challenge timer. You need to dominate 50 events altogether. This means you have to beat the creators score & get 3 stars in that event. E.g. if you beat the creators score in all 5 events in his city, this will be count as 5/50 progress for you.
Most of the scores set by the creators tend to be quite low and sometimes lower than the 3 star requirement, therefore this will not take you very long, even 2-3 days if you pay attention to the hourly challenges.