I tend to feel that if a metamour doesn't want to meet those already in the relationship, then that is a warning sign of something. It may just be nervousness, in which case that can be overcome, but it may be "mistress syndrome" - and I have come across that a few times. "Don't talk to her about me, don't tell her anything about us, and no I don't ever want to meet her". That doesn't "feel right" to me, and it would make me question a lot of the motivations.
In terms of meeting - neutral ground would be very good. A place where talking is easy - i.e. not a nightclub. Also a place where it can be as short or as long as feels natural to everybody.
The last two relationships I have pursued, the three of us met before anything happened in terms of taking the relationship beyond "it's got potential". In the most recent case there were several meetings - I think three. By the end of the third we had each talked about our own experiences, how we felt about poly, and a little of what we were each looking for. After that third, everyone said how comfortable they were, and things went ahead to find out if that potential could be realised in some way. There then followed a few "dates" where she and I found out that yes, there really was a potential for something magical.
Then the three of us sat down again and had a long, log talk about wants, needs, likes, and we formed the basis of what has turned out to be a two and a half year (and still going strong) relationship. Most of the fundamental stuff we discussed at the start is still working and still in place, I am happy to say.
Everyone felt involved, everyone felt that they could have their say and be listened to and respected by the others, everyone understood exactly what it was and what it wasn't, and what areas of concern there were.
"Listen, or your tongue will make you deaf." - Native American Proverb