Great question, Jyo. Some authors just want to keep their plots a secret until the whole thing is done. In many cases, the novelist has a red herring or plot twist in mind, and giving it away beforehand would ruin the plot for readers. In other cases, the author just prefers not to reveal anything about the plot until the novel is finished, maybe because he's hoping you'll read the finished work, maybe because he's uncertain of the quality of his in-progress work and how it will be accepted in rough draft form, or maybe just because the plot isn't solidified for the author yet, and he's afraid that if others read it, he'll be stuck with his first draft. There are some authors who fear that their plots will be stolen if posted online in any form, and the risk of that is a matter of personal opinion, as well - you are correct that if you make your work group-only, only the 75 members of this group can read it; I personally don't think it's a high risk, but respect the opinions of others who feel the risk is too great. It's entirely up to you whether you share your in-progress work. I personally like to have the support of peer authors while my work is in progress, to get feedback and help me shape a successful novel. Again, it's strictly author preference.

Good job getting ahead!