If an ally stands in the space of a prone character then the player may shift when standing up as a free action. My players have used that tactic a couple times now. But if you want to stay prone then you can't
PHB p.292
Stand Up
If you’ve been knocked prone, you need to take a move action to get back on your feet.
STAND UP: MOVE ACTION
- Unoccupied Space: If your space is not occupied by another creature, you stand up where you are.
- Occupied Space: If your space is occupied by another creature, you can shift 1 square, as part of this move action, to stand up in an adjacent unoccupied space. If your space and all adjacent squares are occupied by other creatures, you can’t stand up.