There are a lot of "spiritual cousins" of Jehovah's Witnesses. Herbert W. Armstrong was ordained a minister by the Church of God Seventh Day. A group that had its origins in the the Second Adventist movement of William Miller. When 1844 came and Jesus did not many Millerites set up their own churches.
Second Adventist Ellen White began to share her "visions" founding what would be known as Seventh Day Adventists. An old yankee sea captain named Joseph Bates said Ellen White was nuts and founded what would be the Church of God Seventh Day. (Both White and Bates picked up the Saturday Sabbath from a small group of New England Baptists.)
One of the major influences on Charles Russell was an old Millerite named George Storrs, who helped found two groups, the Adventist Christian Church and the Life and Advent Union.
From the Seventh Day Adventists we have the Branch Davidians of Waco infamy.
The various Assemblies of Yahweh, Black Nationalist and White British Israelite factions, also trace their origins to the splintering of the the adventist movement.
There are a myriad of churches that share the same basic recipe as Jehovah's Witnesses. It is not that they studied with the Witnesses or had ever been part of the Watchtower. The common denominator is William Miller's Second Adventists. From Miller these groups have developed their own little quirks but all share the same nonsense about being "The Truth" "God's Kingdom is no part of the World" "Unquestioned Loyalty To God's Earthly Channel" and that "The Whole World Is Ensnared By Satan"
