Not sure if you have to be level 15 or just unlock it when it prompts but they are located around the map and you can find them on your legend.
The easiest way to play them, though, is press pause, go into the online tab and look for rockstar created lists. They should be listed as Races, Missions, Survival, etc. You'll find all of them right there to host or join.
Hope this helps.
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