You need to be an administrator of 2c8 Portal to do this.
If you want to see what has happened since you last logged in to the portal, go to the section Customer details. Here you can see the Activity log that logs all the activities performed by portal administrators. If you click View more you also have the possibility to filter the activity log based on when the activities took place, who performed the activity, and which user the activity affected. It's also possible to download the activity log as a CSV-file, this is done by clicking the Download icon with an arrow with a line under the arrow in the top right corner.