It is a legitimate shared ride program, and very popular and well-used and trusted in Spain, You have to 'enter' the trip you want to take, (date , departure and destination locations, # people, and luggage, etc.) and registered BlaBlaCar drivers who may be travelling the same route, may choose to 'offer' a ride that matches close to yours, and you can choose to express an interest in their offer, and they can choose to accept you or not. Be prepared, because it's not necessarily door-to-door service, rather they often have specific pick up/drop off location suitable to them/their route, and that may not necessarily be close to your exact locations ..so you may need to taxi/walk etc before and after the ride. I have used it only twice, both within Spain and to Portugal, and it worked out brilliantly, (safe, comfortable, good experience, great price, and I was lucky, .by chance their pick up/drop off points were perfect for my needs.) it's well worth the try. LIke Uber and other platforms, you each create a profile, you can review each others profile, and can also rate your driver/passenger.