This is not so very secret. Tha Spanish section that I attend is on Mondays and French, German and Italian all have their own designated evening of the week.
Visitors, especially native speakers of the languages, are always made welcome free of charge and anyway a year's membership only costs £10 and entitles you to attend all sections.
This circle, owning its own premises and obtaining most of its funds from rent and bequests, is probably unique in England.