If meeting someone for the first time and playing with them in this way, a safeword is essential. Only after you have played with someone a lot and know thier limits really well can you even consider not having one (and even then not the best idea). In this situation I would not agree to serve any of the people present unless there had been a good deal of negotiation first - limits set, an idea of what is expected and at least one way to end the scene if necessary. It sounds like these people consider themselves hardcore and therefore may be dangerous.

I do know some who do play without formal safewords but they still have it in mind that safety is important - the Domme always keeps a careful eye on the sub and stops if essential and the sub knows that they can still call a stop, they just don't have a 'safeword'.

In some scenes a safeword is needed because the sub is likely to be roleplaying. When he is saying 'no, please, stop, don't hurt me!' its all play and he would actually be frustrated if you did stop. However, in many scenes I have been involved in (especially some of the ones where I was filmed) I knew that I could just say stop or otherwise express discomfort and the Domme would stop and at least ask me if I was alright.

A good domme knows how to read a sub in terms of thier expression, skin colour, breathing etc and knows when they are in distress and will stop a scene regardless of whether a safeword is said if they think a sub is in danger.