You might like to read "The Anatomy of Friendship". People are basically hardwired to like people they're physically close to and come into contact with regularly, so try to tick those two boxes.

The trouble with an app like this is that if it works and people become friends they'll probably just switch to the lowest common denominator/free platform, e.g. facebook/whatsapp to coordinate, so monetisation could be tricky. I.e. if your app is 100% efficient, people may use it once then not return.

You also have the added challenge of seeding this "two-sided" marketplace (people suggesting activities and those saying they'll attend) in every locality you want to operate in, so you'll need a compelling growth strategy. It also sinks or swims on people actually turn up for events.

Good luck - I do think Meetup sucks and could be far better, but I think you'd need a lucky break/very interesting twist to really gain traction.