The ability to put a team together is a valid skill in a team game

If you lack the ability to put a roster together then you lack a strength If you have no friends on all the time then recruit from the 9 others you play with each game or the tens of thousands on these forums.

Not being ready to overcome the challenge of an enemy premade is no different from lacking the skill to dodge a skillshot. I did a survey once.. not a very great one but a lot of people seem to have less than 200 people on their friendlist to be recruited for matches at any given moment. If you have less than 200 and can't put a team together maybe consider that people on your friendlist don't necessarily have to be your best friends ever. Or they can be, just put them in another second, you have that option in the client, you can even add notes about them.

edit

Oh and btw, the client last I checked had a chat area where you can find teammates and see their name to check them out.. If you are too lazy to do this and get your ass handed to you then at least don't complain that they had a premade advantage, you did that to yourself.