There's a sample file here that concatenates multiple selections from a
data validation list:

http://www.contextures.com/excelfiles.html

Under Data Validation, look for 'Select Multiple Items from Dropdown List'

Bill Foster wrote:
I have an input screen that I want users to pick a selection from a data
validated list. The worksheet contains many columns for simple user input.
However, the more complicated scenario below applies to just one of those
many columns.

If users want to pick more than one selection I'm hoping that upon the user
making a left click of a selection from the validated drop down box or upon
pressing enter (or, say, upon calculation) a macro will delete the validation
in that cell, concatenate the previous selection (in that same cell) and the
newest selection (both text) delimited by "; " place that combined value in
the activecell and then reapply the validation (perhaps by copying it from
the cell below if that is easier then coding all the validation over again).
The new value does not need to be added to the validation list. And, of
course, if someone only makes one selection for a cell, and simply just goes
on to the next cell the code can rest.

Once the user selects a different cell in that column, the process starts
from scratch. If a cell in another column is selected this code would do
nothing.

I've been trying Worksheet_SelectionChange and Worksheet_Calculate subs
unsuccesfully.

Thanks.