The church of JEHOVAH'S WITNESSES is a bonafide organization and, as such, people are initiated into it with baptism. The ancient church also brought in members through baptism and today, most modern Christian organizations also use baptism as a means of joining organized religions.
Catholics view baptism as a sacrament, while most modern Protestant faiths have a doctrine centered around the "priesthood of believers." Catholics generally require authority to perform the sacraments, but in emergencies, the non-ordained may baptize. In cases where there is no water, baptism can be administered with almost anything (there's even a baptism of desire that will work in dire circumstances). Thus, almost anyone can obtain salvation under the Catholic system. The Protestant view is that virtually anyone can baptize, but many ministers will take umbrage if someone wants to baptize their best friend or their children. Baptists and non-denominational sects will recognize almost any baptism done in the name of the Father, Son and Holy Spirit.
The JEHOVAH'S WITNESSES view baptism as an ordinance that only its ministers can perform. That said, their own ordinations are on very shakey grounds as ordained ministers don't tend to go much past Charles Taze Russell and, or course, Mr. Rutherford. Where they got the authority to baptize remains unclear and troublesome. Since God has never actually confirmed that the whole Watchtower thing is anything other than good intentions and a great deal of faith in the transferance of power and authority, new members (and old) must pretty much rely on assumed authority from On High. It requires faith in the faith of the leaders as far as I can tell.
The priesthood of believers rests on an interpretation of scriptures, not as written to the people they were written to, but that they were, in fact, written as a general letter to everyone who believes. It's better than nothing, and is certainly better than having to rely on the Catholics as they see it. Alexander Campbell believed that if you cling tenaciously to the Bible and its teachings, then anyone can baptize. Of course he was a Baptist who denounced Baptist teachings and started his own movement. His associate, Sidney Rigdon, soon became dissatisfied with this view and became a follower of Joseph Smith, who claimed to have received the authority to act in God's name from angelic ministrations. William Miller was more preoccupied with the timing of the coming of Christ, but when it didn't happen, some of his followers found themselves following Ellen G. White, who also claimed visions (though no conferral of authority). It's thought by many that her teachings found their way into some of the Congregational churches and later, into the JEHOVAH'S WITNESSES. Whether it's calling God by name or worshiping on the seventh day, these are pretty much trademark distinctions that tend to be overblown.
As an outsider, I see the Witnesses as just another manmade organization. If you've been baptized and you feel it was done correctly (whether by sprinkling or immersion), I wouldn't sweat not being baptized into the sect unless being a member of it is important to you. I would ask JW leadership, however, just how they make the jump from a manmade church to a divine investiture that, as far as I can tell, has never happened. I mean by this that if a flaming pillar of fire has appeared over the Watchtower Headquarters, or some manifestation takes place, then I wouldn't be so dang sure God was behind the leadership. In the New Testament, I just don't see any one future church mentioned by name.