Yes, Discuss's advantage is potential, it is not all that easy getting started. The options and variables are great to have once its working (can't do a lot of stuff with other forums), but all that flexibility leads to a steeper learning curve.
Pls head over to the Discuss board, we have some users who have been through it recently and can help you out. Me, too. Here is the link:
https://forums.modx.com/board/?board=27
I know quite a lot about Discuss, but I don't know what level you are with code or with MODX, so its better to deal with specific questions or problems, like this one. I am not a coder myself but still know the system pretty well.
For this consideration, you have a bunch of ways. Here are two easy ones:
1. There is an option in system settings to make the forum private, all forum pages will be hidden to non-users. This is something that some people want, usually to force people to register, so its been made as an option. System settings can help with some other aspects, but there are also other easy ways to customize your forum (check the lexicon for example, a unique feature of MODX and a really helpful way to give us control).
2. To go even deeper, you can make the inside pages private, and make the user permissions high enough to see those pages. Maybe my words are not clear, but some forum developers want a private area available only to certain "VIP" users, and your idea is just make the whole thing the private area and all users as VIP...
So, pls ask away as you go forward over in the Discuss forum. That forum has all of the basic bugs you will come across in general setup, so check it earlier to save time and trouble. I tried to document my entire process of setting up the forum as a guide for others and a resource for me.
Cheers and keep in touch!