Is #8 ground good for 100 amps?

Yes, number 8 copper is the correct equipment grounding conductor for a 100 amp circuit. NEC Table 250.122 lists 8 AWG copper, or 6 AWG aluminum, against a 100 amp breaker.

Grounds are sized to the breaker, not to the load current, because their job is to carry fault current just long enough to trip the protection. That is why the ground can be smaller than the hots.

Do not confuse it with the grounding electrode conductor to your rods and water pipe; that one follows Table 250.66 and sizes from the service conductors instead.

One more wrinkle: if you upsize the circuit conductors for voltage drop, 250.122(B) makes you upsize the ground proportionally.

The 100 amp ground wire calculator sorts equipment grounds from electrode conductors so you pull the right one.