Premades should recive one pool of LP distributed between members for victory.
For example, solo player gets 20 lp for win, but premade team with 2 members should get 20/2 lp, so each get 10, 3man would get 7, 4man - 5, etc. I think it would be more fair towards solo players that way, and playing in team would not provide straight forwad adventage.