If they have had it already and/or have immunity thres no issue.
Its if they haven't got immunity that chicken pox in pregnancy can be a problem. I don't know the exact details but it can be fatal for the unborn baby.

I don't really know the mums at playgroup, cos my mum takes DS, and I have no way of contacting them as I don't know there phone numbers, and the group orgnaiser is not answering her phone.