I hve no idea how they were recruited in the past. In more recent times, enumerators are recruited through local authority staff offering to do the work but on a seperate contract. A friend of mine took part in the 1991 and 2001 Census and in the last one she was a superviser. The work can be quite difficult; going to areas where there may be hostility or trying to seek out and persuade rough sleepers to take part.

Best wishes

Jerry