Arduino with multiple buttons - will power be an issue?
Yes, it is safe to connect multiple buttons to ground. The pull-up resistors have a fairly high value - about 10 kΩ and we know from Ohm's Law that 

I = V / R = 5 V / 10k = 0.5 mA


per button. Your USB power is good for about 500 mA.

Welcome to StackExchange.