"We believe in God, the Eternal Father, and in his Son, Jesus Christ, and in the Holy Ghost."
The Holy Ghost, or Spirit, is a someone confusing entity in LDS theology. In the LDS Bible Dictionary, he is simply described as the third member of the Godhead, a personage of Spirit, and it is explained that there is a difference between the power of the Holy Spirit, which is the “convincing witness that the gospel is true,” and the gift, which “can come only after proper and authorized baptism, and is conferred by the laying on of hands.”
Its assumed that he's a spirit-child of Elohim, much like Jesus and ourselves. Someone he was also promoted to Godhood without first receiving a body and earning exaltation, but there is no story of faithfulness and selfless volunteering or any other such thing like there is with Jesus. He has no story, in fact.
The Holy Ghost works mostly through feelings for a Mormon. They "feel" something is right or true, and believe the feeling comes from the Holy Ghost. This is how most Mormons "know" the church is true--they've had a witness of truth through feelings.
The Holy Ghost in Mormonism can also leave an "unworthy" member, even though you might think that's when the guidance would be needed the most.
In Christianity, the Holy Spirit is much different and much more potent. He is another aspect of the Trinity--he is God. He has many purposes, but testifying of truth through feeling alone is not one of them. I'm not saying that feelings aren't important and that the Spirit can't touch us through feelings at appropriate times--he's God, after all, he has that capability--but determining feelings based on truth alone is not logical nor Biblical.
1 John 4:1 "Dear friends, do not believe every spirit, but test the spirits to see whether they are from God, because many false prophets have gone out into the world."
We are told to test spirits. The Spirit may touch us through feelings, but if our feelings then do not align with other apparent and tested truths--the existing scriptures, scientific evidence, logic, etc--then it is most likely our feelings that are wrong, not the evidence at hand. If feelings alone were the standard of truth, most religions and faiths would be true, no matter how crazy they are.
The Spirit isn't just some define testifier of truth and producer of good feelings and moral guidance. He is God living in believers. We are told to walk by the Spirit--in other words, letting him lead and shape our lives as God wants it to be, to be not only the best we can be, but also to help those around us and be an example of a life transformed in Christ. Doing this yields the fruits spoken of in Galatians.
5:22-25 But the fruit of the Spirit is love, joy, peace, forbearance, kindness, goodness, faithfulness, gentleness and self-control. Against such things there is no law. Those who belong to Christ Jesus have crucified the flesh with its passions and desires. Since we live by the Spirit, let us keep in step with the Spirit.
No priesthood conference is required to receive the Holy Spirit.
Matthew 3:11 “I baptize you with water for repentance. But after me comes one who is more powerful than I, whose sandals I am not worthy to carry. He will baptize you with the Holy Spirit and fire.
John 15:26 “When the Advocate comes, whom I will send to you from the Father—the Spirit of truth who goes out from the Father—he will testify about me.
Acts 1:8 But you will receive power when the Holy Spirit comes on you; and you will be my witnesses in Jerusalem, and in all Judea and Samaria, and to the ends of the earth.”
Acts 2:4 All of them were filled with the Holy Spirit and began to speak in other tongues as the Spirit enabled them.
Acts 10:47 “Surely no one can stand in the way of their being baptized with water. They have received the Holy Spirit just as we have.”
Belief in Christ is all that's required to receive the Holy Spirit. Belief in Christ is all that's required for anything that God wishes for us, because all we need is Christ.