On 2013–04–13 Robert Blackstone wrote:

> (I tried your liitle code snippet but ConText would not have it.) 

He's missing a closing bracket on the \externalfigure[mill] line.
Here's a complete example:

\setupexternalfigures [location=default]

\defineitemgroup [subfigure] [lefttext=, stopper=]
\setupitemgroup  [subfigure] [a, text]

\starttext

\startbuffer
  Dutch scenery
  \startsubfigure
    \startitem A cow  \stopitem
    \startitem A mill \stopitem
  \stopsubfigure
\stopbuffer

\startplacefigure [title=\getbuffer, list=Dutch scenery, reference=fig:scenery]
  \startcombination
    \startcontent \externalfigure[cow][width=.5\textwidth]   \stopcontent
    \startcaption a)                                         \stopcaption
    \startcontent \externalfigure[mill][width=.45\textwidth] \stopcontent
    \startcaption b)                                         \stopcaption
  \stopcombination
\stopplacefigure

A cow is shown in \in{figure}{a}[fig:scenery] and a mill in
\in{figure}{b}[fig:scenery].

\stoptext

Marco