Jehovah's Witnesses?

Once again, the trinity is denied.  Jesus is not God, but instead they say he is "a god" (small g).

The Bible (and the protestant Christian faith) teaches us that Jesus is equal with the Father, that he and the Father are one.

JWs (Jay Dubs) twist scriptures to suit their perspective rather than informing their perspective by the scriptures.  Be wary of any group that does this, but remember that many who belong to such groups are manipulated to believe for many different reasons.

Here is a heap of resources.

JWs in a nutshell (from www.carm.org)

Mormons?

Mormons have the Bible, but they also have other books which they follow if the Bible contradicts them.  This means that Mormons and Protestant Christians can't be the same and don't serve the same God.

The God of the Bible is Triune/Trinity in nature.  Mormons have multiple Gods.  On this one aspect alone there is a serious difference but there are many.  Mormons believe that their good deeds count towards their salvation but for Christians salvation is by faith alone.

Mormonism in a Nutshell (video from carm.org):