For this glitch you're going to need a couple things, you will need a warlock and you will need the exotic chest piece voidfang vestments that is upgraded. Now I'm assuming this works with the hunter and the lucky rasberry; but I have not tested it. (Can someone please confirm)
1. Throw your grenade(s).
2. Go to character menu.
3. Switch your primary weapon to something else and back to what you had.
4. Exit the menu and your grenade(s) will be full.
This is very useful for the raid when pushing the Templar off.
The reason this works is because voidfang vestments has a perk that allows your grenade(s) to regenerate when you respawn and switching your weapon somehow is making the game think you have respawned, therefore replenishing your grenades.