Note that in order to share a file, you need to have already accepted the person as one of your contacts. This procedure is explained in my previous Slideshare tutorial about How to add Guest Users to IBM Connections,
This tutorial explains in simple terms, how to share a file with users. Everyone, even guest users, can share files (though guests clearly have lower limits than fully registered users).
If you do find yourself running out of room, consider creating a community to share your files in. Communities don't have file limits and aren't counted against a user's personal storage quota.
Feel free to use this presentation in your own organisation if it's relevant;