org-agenda-file-regexp

org-agenda-file-regexp is a variable defined in `org.el'.
Its value is

"\\`[^.].*\\.org\\'"

Documentation:
Regular expression to match files for `org-agenda-files'.
If any element in the list in that variable contains a directory instead
of a normal file, all files in that directory that are matched by this
regular expression will be included.

You can customize this variable.