I'm gonna go with this theory because the tribe seems to accrete members over time that needed it for a variety of reasons - some seek the tribe out, some wander in, some are found in Kidaha's exploration, etc. etc.
Going already by the first story the village seems to be more or less the recognised regional repository for women who for one reason or another don't really "fit in" with mainstream society, which makes a lot of anthropological sense given how many real "primitive" societies have dealt with such outliers.
Being closely associated with the volcano, no doubt a major regional landmark loaded to the gills with mystical symbolism, cannot but help make the community widely known through the regional exchange networks and further add to its special status and duly help attract immigrants.
On the whole I get the impression Mira got the inspiration for this series from reading some proper, if popularized, Stone Age scholarship and related topics.
