The "list of composers" is generated by mp3tag in the drop-down list if it finds more than one composer. You then have the coice to either keep the variety or set it to one already entered by selecting it from the drop-down list or entereing a completely new one.
All data that you edit in the tag panel and then save it is stored in all the selected files of the file list.
If you select only one file then the data will be written only to that one.
So: to get a "list of composers" you have to enter them first more or less one by one.
So in a way it would be a good idea to select all the files that should get the same composer, enter the name in the tag panel and then save the data to the selected files.
If you have a number of tracks where composer and artist are the same person, you can apply an action that copies the artist to the composer tag. there is an example in the faqs.
(Please not that in the new version the field band is now called albumartist)