what functions add to GB usage on net
As a direct answer to the post topic, everything! Every activity by every person connected to your cellular device uses data.
All the video ads you see running in the side bar eat up data. Surfing page to page eats data. The things that eat data the most are video related whether that's watching a YouTube video or playing games where the graphics are constantly streaming (all live action).
Large downloads such as system operating updates (especially with that many people/devices connected) as well as an software downloads or updates like updating a Garmin GPS (2+GB there alone).
If you like sending e-mails with pictures of your last adventure, an unedited picture send from your phone or computer can be 2MB each. If you have any kind of cloud storage, these are updated automatically along with the other changes made. I screwed that up when I needed to send a large file and signed up for Dropbox. I did not realize that Dropbox then started uploading all my pictures from the phone and computer. It got very ugly very quickly and cost be a bunch of money (additional GB's).
If this is going to be an ongoing situation I would suggest that you find a high GB provider such as Millenicom so you have a fixed fee for up to 20GB and you could supplement that with you current cell plan should you find that to be necessary.
