Christadelphians are a small cult, founded in in the United States by John Thomas in 1833. They were also known as the "Brothers of Christ" and the Thomasites. This group may be the parent group of the Jehovah's witnesses. Many of their doctrines are the similar. 
They teach that their founder, John Thomas, brought to light through his interpretations the "lost" truth, which had been corrupted with apostasy for centuries and without his interpretation, the bible cannot be understood. Their doctrines are as follows:

1. You must become a Christadelphian and be baptized by them in order to have a chance at salvation.
2. Jesus was the son of God, but not God in the flesh (denying the Trinity) 
3. The Holy Spirit is the radiant energy/power of God, not 3rd person of the Trinity, and he does not indwell the believer. 
4. Jesus was not  born sinless. But we must achieve sinless perfection, like Christ did in order to save ourselves. This is referred to as the "God Manifestation". 
5. Baptism and other works are necessary for salvation.
6. Hell is not eternal. The wicked are annihilated. Except for the resurrection there is no type of life after death.
7. They are not to fellowship with any who do not hold to their doctrines.
8. Jesus Christ was just an human being. His death on the cross was not substitutionary, does not atone for are sins, and is not sufficient for our salvation. He only died so that we could have an opportunity to be like him, he was only a representative, not a savior. 
9. The soul is not immortal, it ceases to exist after death. 
10. The Devil, fallen angels, and demons are not real. They are just a euphemism for sin and evil men. 

They have various by-laws and standards regulating behavior, conscious objectors, no voting in elections, no union membership, no self defense, no employment as a police, or employment where military goods are manufactured. Children discouraged from playing after school sports or becoming members in boy/girl scouts, no membership in any fraternal organizations at all. Marriage outside the Christadelphians is ground for disfellowship, as is any disagreement on any of the above. They have no organizational hierarchy, and each ecclesia (church) is presided over by part time lay ministers, who do not have any formal training other than studying for baptism, which is a lengthy trial of interrogation by other Christadelphian brethren. The women are to keep silence in the church, but may teach the children and each other, are to cover their heads and flashy dress is discouraged. There seems to be a greater tendency than average to utilize holistic health practitioners, (somewhat reminiscent of Christian Scientist) although there is no prohibition against blood transfusions, they have been known to discourage it.

Information taken from "The Constitution of the Glendale Christadelphian Ecclesia", Glendale, California, 1972.