This all depends on what type of deal you agree(d) to with the artist, and what you _
GET IN WRITING_! (I can't stress that enough). "Back End Deals" with independent artists usually don't work-out so well, but at least get an expressly detailed agreement in writing. It can even include you getting paid a certain amount (which would have/should have been your fee) after a certain number of sales (say, 1,000), separate from any points/royalty over-ride you may have negotiated. Make sure you cover all possible bases-- define what a "release" is (including CD's, LP's, tapes, Mp3's, Wave files, any possible video formats, etc., etc.).
As to the copyright filing issues (BTW, it is "Copyright," as in "Copy Rights," not "copywrite," which is writing copy for news releases or public service anouncements or advertising, etc.): There is no one answer, because there are so many factors involved. Are you planning to retain rights, or are you selling them outright (highly unadvisable, but that is what so many people seem to be doing nowadays)? Is publishing/co-writing part of your agreement? The problem is, once his or her lyrics are married to your music, they become one piece, and _both_ of you will have rights to _both_ parts (words and music), so think carefully about what you will be doing.
One way would be for you to file copyright on your beat compilation now, then pull the one song that will be for the artist/co-writing situation with what is called a "change form." You can find out about the specifics at the Library of Congress site on-line;
www.loc.gov .
I suggest that you really get your knowledge of copyrights, publishing, and general music business together, if you are representing a group of people in your production team, and perhaps have an attorney draw-up some contracts based-on different scenarios that you might be selling/leasing/licensing/exploiting your music. It is so easy to get ripped-off one-thousand ways in the music business, but you don't want to avoid good opportunities due to fear or lack of knowledge either. A couple of good authors that have written great music business primers include Donald Passman, Kashif, and Moses Avalon. Moses Avalon also has an artist-advocacy website. Better get crackin'.
GJ