Which Christian denominations do NOT believe that Jesus existed prior to his conception?

Iglesia ni Cristo (Church of Christ) has ~2.3 million adherents and is the 3rd largest religious group in the Philippines.

"But we do not subscribe to the belief that Christ is a God-Man or both God and man. He is man in nature according to His own testimony (John 8:40) and the teachings of His Apostles (I Tim. 2:5; Matt. 1:18)." The Lord Jesus Christ

More specifically, a letter to the editor of Iglesia ni Cristo's publication God's Message asked

"Why is it that your religion does not teach these three basic doctrines (divinity, pre-existence, incarnation) about Christ?"

and the answer was in part

"What is spoken of in John 1:1, 14 as being with God in the beginning is the "Word." Hence, in order to understand the real message of John 1:1, 14, we should first clarify the meaning of the term "Word." Does it really refer to a 'pre-existent Christ' as others allege? No. The Holy Scriptures prove instead that the "Word" refers to God's "promise" to send His Son, which He "announced" before[.]"

Christadelphians number ~60,000 adherents.

"Jesus Christ is a man, Christadelphians say, not God. He did not exist prior to his earthly incarnation." Christadelphian beliefs and practices

Church of God, General Conference has ~5,000 adherents.

"Jesus is the Christ. Existing from his mother’s womb, he is the virgin born son of God (Luke 1:34-35, Gal. 4:4)." Our Beliefs

I'm sure there are more, but verifying non-English churches becomes tricky.

BiblicalUnitarian.com and the Revised English Version Bible translation (including commentary and appendices) is by the Spirit and Truth Fellowship, a non-denominational ministry. I do not know the number of people affiliated with the organization (I presume it is small), but the resources they provide seem the most comprehensive and easily accessible of Biblical Unitarian groups. They hold that Jesus pre-existed only in the sense of being part of God's plan. I believe this would qualify them as denying Jesus' pre-existence per the OP.

"The Jewish worldview was that something planned (that is, foreknown in the counsel of God) existed notionally or ideally, but not yet in actuality upon earth in our experience. [...] When this paradigm is applied to those texts which seem to our Western understanding to imply that Jesus personally did consciously exist (as God) before his physical appearance on earth, confusion disappears." Jesus' Pre-Existence - Literal or Ideal?

Christian Science (Church of Christ, Scientist) has ~100,000-400,000 adherents, and might seem to qualify as it holds Jesus is a man.

"Jesus Christ is not God, as Jesus himself declared [...] Jesus is the name of the man who, more than all other men, has presented Christ, the true idea of God[.]" Christian Science Beliefs quoting Science and Health by Mary Baker Eddy

Christian Science isn't a creedal denomination (see here), but the founder Mary Baker Eddy held to universal pre-existence in some sense, referring to "man's spiritual preexistence as God's child". As the article continues, commenting on this:

"Christian Science teaches that man's identity, his real being, is the reflection of Spirit, and therefore wholly spiritual, and that his existence as the child of God, who has existed with his Father-Mother God "from the foundation of the world," is eternal.

According to Christian Science, when Jesus said "Before Abraham was, I am" the 'I am' was referring to a universal Christ of man's true spiritual nature.

"When Christ Jesus made the startling declaration, "Before Abraham was, I am," he not only referred to the eternality of universal Truth, but also to his own pre-existence as the individual son of God, an individual instance of divine reflection. It will be readily seen that these words apply to all the individual sons of God, just as they did to Jesus, the Christ." Before Abraham Was

So, it seems this denomination believes Jesus pre-existed, as all humans have in their 'real being'.