1) It's the other way around - they're connecting to the hotspot, it's not connecting to them.
2) If the hotspot is on 5GHz (802.11a or 802.11n using 5GHz) and the PC only has 2.4GHz (802.11b/g or b/g wifi), it can't see the signal.
That's a guess - only being there and measuring things can give you a definite answer.