Should you use multi-site or multiple Drupal instances to host different content for subpath sites?
Subsites should work for Drupal 7 as well (assuming this is what you're working with).  I've found these instructions helpful in the past: http://drupal.org/documentation/install/multi-site

If the company subsites you are running are under one parent company where there is one body of management (a common decision maker), then subsites makes sense.  If they are different companies which may want very different functionalities for their sites, then it may not be wise to combine them into subsites which rely on the same code.