I am looking for an idiomatic expression (something similar to "by choice" or by "one's volition") that would mean 'by volunteering.' 

Sentence: 

> 
> 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.