You must first allow signatures on your forum by going to: Admin Panel>>Users & Groups>>User Options>> and tick the box "Allow Signatures".

I wouldn't recommend adding signatures to your members accounts unless they want one. That is not a very respectful thing to do without there approval. Its invading there space. However, you can go to Admin>>Users & Groups>> Then click a users name and add the signature in there HTML signature box, and tick the box "Always attach my signature" then save the settings.

You'll have to do that for each member you have if you want to add a signature for them. And the signature will NOT show up until they make there next post after you add it if they wasn't using one before. And the only the posts that will show a signature are the posts that had a signature attached to it before or new posts they make. If someone has not used a signature in there posts before then those particular posts will not show a signature ever. Only the new posts after the sig has been added will show the signature if they never had one before. The posts that had a signature attached before when they posted will show the new signature.

thanks alot man