Complex sentence: They watch and criticize their former group and provide information and their perspective to the public they state is not disclosed by these groups, purportedly to enable current and prospective followers to make an informed choice about joining and staying with the group.
Split into simpler sentences: They watch and criticize their former group and provide to the public information and their perspective which (they state) these groups do not disclose. Such' critical former members often have the stated aim of enabling prospective or current followers to make an informed choice about joining and/or staying with the group.