Complex sentence: The company disputes that they were expelled stating that they had resigned from the organisation and informed AIPP in writing prior to the alleged expulsion for changes to membership procedures which they found unacceptable.
Split into simpler sentences: The company disputes that they were expelled stating that they had resigned from the organisation and informed AIPP in writing prior to the alleged expulsion. They decision to resign was based on changes to membership procedures which they found unacceptable.