..in my own experience of the local congregation the ones who came in from outside were either old, loners, disabled, widows/widowers, or transitory, ie they'd come for a while then drift out, only to be replaced by fresh ones,...rather like a social club than a religious meeting.( In fact during my first year or so in I was swamped with meals out, cinema outings, etc etc almost as if to help me avoid any genuine ' smelling of the coffee'....)
.to back this up when I first went, in 1983 , the congregation numbered about 90, after approx 10 years it was decided to form another congregation, (probably to allow born in teenagers the opportunity to gain 'privileges'/reach out ) so both had approx 55, when I left 3 years ago each cong numbered about 75, so total of 150 from original joint of 90....hardly massive growth, even allowing for comings and goings..
There always seemed to be babies or babies on the way, as soon as a young couple married "in the lord" within a year or so they'd be reproducing...very few if any normal, run of the mill, mr and mrs smith, average ,worldly couples/single ever came in,...