The poly/group/add-in world is a complex one.
For one the candidates are fewer then in the regular dating/BDSM one-to-one relationships and there is no or almost no pro involvement.
As a results people that negotiate in such relationships want to be absolutely sure about what they will step into because they have usually some longer term expectations. Furthermore there is an extra obstacle. You'll often find couples at the start-up of such multi-people set-ups (not talking parties but relationships). That introduces extra tension. Did BOTH want this or is one of them doing it for the other? Are the potential play-partners a threat for the existing relationship?
Translated: Will the other woman take my man. Will the other man be a better Dom/Top/ect then me? He likes her but she doesn't really like him (or the other way around).
So in negotiations all that information isn't often volunteered. It like a Job interview, they prefer to write you or not tell you anything at all if you are not the candidate. It is the same here. Few will say: We think you are not the one to play with us/me because either that may come across as embarrassing or it will entice the "why" question.
So it mostly comes down to : We'll call you.
Very seldom are the negotiation topics (as in limits and boundaries) the problem. The field is so large that there are always common grounds. It is in 95% of the cases a like or don't like at one side or a 'feeling threatened'. And safety, yes safety is always to be considered. But safety begins with personal disclosure, who am I, who are you, exchange data (pics), do real-time communication which cannot easily be faked, etc. That is where 50% of these negotiations go down due to misrepresented people that cannot enter that last stage because you'll find out she is a he, they are at the other side of the world, playing teenagers or people that love they idea but know they will never meet but cant' say it otherwise you hook off early.
So how do you solve that. Very simple.
1) Offer anonymous (ID-less, no phone #, no address needed) communication that forces the person to show itself while you must show yourself too. Skype alike communication offers that privacy and these days no one that has a computer can find a reason not to Skype if they are really seeking and are who they claim they are. If you have the time for BDSM play relationships then you have the money for a cam otherwise you are better off taking a second job first.
2) In such communication you can See and Hear the person(s) and those that fall in the categories forcing them to hit & run will not enter that option. They'll find reasons not to and you'll cut down on them, not loosing your time. May there have been some real candidates in there? Yes, but that is collateral damage.
3) Ones you know you are dealing with real people and had mutual disclosure the security risk is also lowered. Agree that both parties can at some point step out, no questions asked. That will cut down negotiation time because people will no linger once they came to a conclusion that it is not a match.
4) Don't take rejection personal. In many cases it isn't. Some people engage in something and then think "I shouldn't have done that". Couple have that. 90% he asks, she agrees to please, then after some reflection one backs out and the party they were negotiating with remains hanging.
5) In a pure BDSM context, stick to the roles from minute ONE! Let ONLY the Masters/Doms/Tops negotiate for their side. A lot of interesting play and sensation is ruined because because many think you need to negotiate as equals. That isn't the case, on the contrary. The Doms are equals and their subs obey. That doesn't mean that behind the scene they can't consult.
So bottom-line: There is no reasonable time to negotiate. Some people decide fast, others need extra privacy and security guarantees, etc. I would therefore say that after a few chats/mail exchanges one that is serious should be prepared to Skype, if not there is a big chance there is NO intend to carry in but virtually.