Google Script for Spreadsheet - how can I prompt the user to select a range?

Google Script for Spreadsheet - how can I prompt the user to select a range?

I've been doing a lot of work with Google Script to automate some peer
evaluation processes.
One result is a spreadsheet with a lot of review data that I need to sort,
group and summarize. The script is working fine for the hard-coded sort
column.
But I really want to be able to sort on different columns at different
times, and I'd like the sort/group/summarize script to prompt the user to
select the sort/group column at runtime.
I've explored the UI stuff a bit, but I haven't seen how to prompt the
user to select a range (or column or anything else) on the sheet.
Thanks in advance.