If you wish to invite somebody to Belgium, and you wish to be financially responsible, you must sign a pledge of financial support. You can obtain this document at the town hall where you are registered.
If you are registered with an Embassy abroad, you can obtain this document at the visa section.
Be aware: only the original document is valid. This must always be submitted at the time of the request for visa.
For more information, visit this website (website under construction, not yet available in English)