You get a Friend Bonus whenever you fill the friends' bar on a Hangout. Those bonuses award payout multipliers for that Hangout, as well as a small amount of Tapes and/or Cards.

The green meter indicates your progress towards the next Friend Bonus after making more friends. The orange box displays how many friends you can make at once how much it costs.

If a Hangout's meter is completely full, with MAX above it, it means you've already earned all its bonuses. Keep in mind that you can still make more friends to increase that Hangout's payout or to complete certain goals.