I hava blog and would like to allow guest post on it. User registration would be better and easy for this. But is it safe to allow it. I dont know much about this. Will there be any security issue if I do so. What would me measures I should take. Any plugin, or any other suggestion welcome.
There are some free plugins out there that help you manage users in Wordpress. They allow you to do things like require new posts to be admin approved, limit the actions that a user can take (such as not uploading images), and remove various elements within the backend from view for different roles (don't let users have viewing access to the plugin panel, theme panel, etc). These help secure your WP install against unwanted tampering, minimizing your risk.
Allowing open submission without checking the quality of content might be a bad idea especially if you use Adsense etc. Spammy duplicate content would be the ones which you would probably receive. Even if you want to open registrations, make sure to check with copyscape and proofread before approving the articles. Do not give a random user author permissions in wordpress. At most a contributor.