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There is an additional column in Schengen visa for comments (or attach a cover letter). You can just mention like this. Your main purpose is tourism. You will be meeting friends who are there, if you get time. No need to provide details of friends, unless you plan to stay with them for any amount of time.
Note that you have to provide a detailed itinerary, like where you are staying, how you are traveling inside the Schengen area, modes of transportation (with tickets) and dates. A cover letter with clear itinerary will make everything clear and will increase your chance to get visa and avoid confusion.