Should I join sgi?
If you Google on SGI, you will find many unsettling accounts about the group.
On reddit there is also
A friend of mine is into SGI. I ended up going to one of their meetings.
I asked about the cult accusations there. A person who had been with SGI since the 70s got a pained look on their face, as in remembering something they would rather forget. That person told me that SGI was created in Japan in the troubled aftermath of WWII and that people did extreme things that the group came to regret.
During the rest of the meeting most of what people had to tell me was how they got many things they wanted and solved mundane personal problems once they started chanting a mantra every day.
It sounded very much like the idea underlying the "law of attraction" nuage stuff. Do something in your mind, have the new job, the new car, automagically appear in your life.
The people at the meeting did emphasize that SGI was so much more than that, but they didn't go into it.
Based on what I heard at that meeting, and having read a book by the SGI founder that my friend gave me as a gift I don't think SGI is Buddhism.
Buddhism is based on the core idea that every person experiences unwanted changes in their lives that will make them unhappy, and that people can learn to suffer less by practicing a combination meditation, cultivating wisdom, and behavioral restraint.