I am looking for an idiomatic expression (something similar to "by choice" or by "one's volition") that would mean 'by volunteering.'
Meeting in cafes (or sometimes in private apartments by volunteering), we would discuss a wide range of subjects.
I mean to say that in addition to cafes, which were our regular setting for meetings, some members would volunteer to host us in their apartments.
P.S. The expression "on a voluntary basis" a) sounds a mouthful to me, and b) implies a sense of regularity which I don't mean to imply in the sentence. Thanks.