I place my introduction board either next to the General Board, or in the information category. Those two places I expect a "New Members" board to be placed.
I normally try to place it near the top of the index with the announcements forum and suggestions and important things like that. I want it to be easily seen so that nobody misses it so that they can all get a warm welcome.
I think it's important to place the introduction board at the top of the forum so it cannot be missed. It is also helpful to reach out to new members by sending them a quick PM introducing yourself and reminding them that they can introduce themselves by starting a new thread in that section and including a link in the private message.
I usually put it underneath the suggestions and information forums. However, I prefer to put this category at the bottom of the index, as opposed to the top like a lot of people. I just don't think that category is as important as the "on topic" sections, which is why I put those sections first.
I place my introduction forum at the bottom of the "Announcements" category, which is the first thing you'll see when visiting. I've always had it like that and it feels weird not to have the intro board somewhere near the top of the forum. 😛
It's placed in the same category as our general discussions and forum announcements. There's no reason to expect a member to register for the sake of introducing themselves, and then leave. It's best to just get down to posting in the main forums.