What is the cheapest option to fetch a person over a long-distance with bicycles? I need to fetch an outsider from about 20km far away and I want to use my bicycles. I have used roller skaters with a bicycle to fetch my friend, I let my friend to use the bike while I will stuck to the back at the destination -- it is very cool and convenient (light weight) when you know your friend but doing it with unknown outsider is not be the best option because it requires trust between the participants. My next best option is to use my foldable bike stuck to my back-bag and my diamond-framed bike but the foldable bike weighs about 10 kg. I have been considering a number of options such as tandem, an unicycle and side-cargo thing but many options tend to be expensive. I don't want to use rack based sitting approach because it would damage the rack.

How do you fetch your friends or outsiders over long distances when you need to carry the transportation equipments to one-direction alone and then allow the friend/outsider to use the equipments to move to the other direction with you? 

If you are suggesting a thing such as tandem that may be too expensive, let me know whether you could hack your standard bike such as diamond framed bike to a tandem so that I could get it cheaper.
When I drop my wife off at the train station, we use a folding bike and a trailer. She rides the folding bike there, and her suitcase rides in the trailer behind my bike. However, while inexpensive used folding bikes can be found, getting a trailer may be too expensive. 

You'll either need to carry someone on your bike, or find a way to bring along another bike. 

You can hold onto another bike while you're riding, wheeling it along next to you, but that's not very safe (especially for rides of any distance). Since you've indicated you don't want to carry someone on your rack, I would consider buying an inexpensive beater bike and a truck bed mount, attaching the mount to a bike's rack to receive the front fork of the bike you're pulling. (Here are instructions to make one of those inexpensively, scroll down a bit on the page and you'll see a commercially produced version of the same device.) 



You take the front wheel off the towed bike and the bolt shown goes where the axle of the front wheel used to. Works like this:



You can ride with one bike behind the other, strapping the front wheel of the bike you're towing to the side of your rack, or just carry it. You'll also have the empty rack for the trip back.