Don’t go by volume, go by temperature. Put the volume where you want it but keep going and feeling the top to see if it’s able to lose the heat. I’ve had a few parties with my nova. Warm is ok but if you consider it to feel hot, back it up a bit.
Assuming you mean safe for the amp as opposed to safe for people’s hearing, it fundamentally depends on the speakers and the load they present to the amp. But if you hear any distortion setting in, notably on the loudest bits of the music, then it is turned up too much - distortion is a warning sign: If due to the amp being pushed into clipping then the speakers may not last long, ditto if due to the speakers being overloaded. If the Star has power amp protection built in, which I understand is the norm with Naim products, then damage to the amp from overloading seems unlikely - though in common with some of their amps it might switch itself off for a while, which you wouldn’t want during a party. That is where the guidance some people have offered regarding case temperature, but the big question there is what temperature would correspond to the trip off point - why not ask Naim that, and check the case temperature, with your hand if you know roughly what feel corresponds to temperature, or using a point contact digital thermometer or low range infrared thermometer? How loud it would be and how quickly reach the trigger temperature would depend on air temperature around the unit - which may be higher (and thus more limiting) if in a crowded room full of moving people on a hot day than doing a test on your own today.
How loud the highest level would be then depends fundamentally on the sensitivity of the speakers.