Hi Karen,
Thanks for reaching out!
This narrowness is the result of the number of columns being displayed in your admin view. You can fix this by hiding some of those columns so that the visible ones have more room to show up. You can do this by checking and un-checking checkboxes in the “Screen Options” for this admin page. To learn how to do that, read the “Screen Options” section of this page → https://codex.wordpress.org/Administration_Screens#Screen_Options
Once you get the column sizes showing correctly, if events that have attendees are not showing anything in the attendees column, then there might be a conflict with your theme or another plugin. You can test this by running through the steps outlined here. Please note that a line will show up in that column for events that do not have attendees.
Let me know what you find!
Thanks,
Jennifer