Complex sentence: Local Youths (LYs) from Japan join PYs in the Japan-ASEAN Youth Leaders Summit (JAYLS), where they are separated into groups and take part in Discussion Group Activities.
Split into simpler sentences: Local Youths (LYs) from Japan will join PYs in the Japan-ASEAN Youth Leaders Summit (JAYLS). They will be separated into groups and take part in Discussion Group Activities.