Answer: First of all we check our friends, but we do it once, at the very beginning. After making that check we have to rely on our unity.
It is called “Mikveh,” which is a special water reservoir for the purpose of purification. When the Light becomes revealed inside our unity, its force cleanses and purifies us. However, if a person is completely unfit for it, then it cleanses others away from him. It influences him in a way that casts him away. This is the effect of the force of cleansing: It either pulls us inside, or throws us to the outside.
Therefore, one should assess his friends only by ascending above reason and caring only about how to strengthen the common force of unity in the group. Then this force, the Creator, will clarify: Who is outside and who is inside.
Those who are really trying to reach unity are pushed inwards by this force, whereas those who don’t merit it yet are thrown back. A person who is unprepared cannot go through the burdening of the heart and a descent. Instead of giving him extra forces, his states will force him to break away to the outside.
From the 1st part of the Daily Kabbalah Lesson 10/27/10, “Make Yourself A Rav, Buy Yourself A Friend”