I don't think simply adding a community will hurt rankings. Indeed, adding new content should increase traffic if the site has good existing rankings and the ability to rank new content quickly.
A few suggestions:
- Keep the community free of spam, problem links, etc.
- Keep the main topics relevant to the site. Segregate off-topic stuff into one area if you can, and keep that out of the index.
Your community may even help the overall site by increasing the level of user engagement as measured by pageviews per visit, time on site, etc.