It honestly depends on the betta, some are fine with them some are not. I originally had three in with my betta, he did okay with the biggest of the ghost shrimp, but killed one and tormented the other till it jumped out of the tank. I've heard of many people who have had bettas fine with them though. It also depends on how territorial your betta is and how long its been in its tank, if it thinks that the shrimp are invading its space then it may eat them.
All in all ghost shrimp are fairly inexpensive and so it would be worth a shot. Worst case scenario is that your betta would either end up eating them or if your worried about them to much, put them in a smaller tank of their own.
peace. love. & bettas.