The Fish River is a trail that needs to be booked well in advanced due to the seasonal aspect as well as the water levels in the river. While they do allow you to tag onto smaller groups I wouldn’t advise it. 4 - 5 days is a long time to get along with people you might not necessarily like. Maybe wait another year and rustle up a few buddies (3-4 people)?
Furthermore, i doubt that there is any space available for 2018 as we really struggled to get our confirmation for 12 people.
the below is a great article which covers the trail as well as the booking information:
Here’s another, more recent one:
Best of luck