Need to be able to rankup using PEX!

I am making a factions server that uses ranks for extra permissions (like kits etc.).
I am using PEX for permissions, and essentials for general stuff, and I also have vault (but I don't use it).
I would like people to be able to rankup, be moved to the PEX group and consequently get all the permissions stated in the PEX group data. Essentials handles my kits.

Thanks for any help,
Olsyboy