Both Clergy and the TGs are incredibly advanced individuals, whose role is, well... to be priests.
Similarly to priests of the Gods in ancient times, they communicate directly with the Gods, work on steering the community in the right direction, guide members, and perform more direct spiritual or astral work for the ToZ.
High Priests specifically are essentially enlightened souls already, Divine-like, and are here as nothing short of Gurus with the direct task to enlighten and advance humanity, like the many Divinely sent guides in Vedic mythology.
One could probably eventually advance to becoming a Guardian or Clergy, but this is something that the Gods ultimately decide for the ToZ, and the title is an incredibly serious path. Effectively, you can't just "decide you will become one", it is having a very specific soul Dharma.