This is one of the most important questions because your answer will determine how hard your setup process is going to be. There are basically four options available to you for your member interaction inside of your website.

1. Forum - If member interaction is will be a key part of your membership website, a forum is becomes a must.  There are a many forum software options that you can choose from, each with their advantages and disadvantages and each with their own level of complexity in setup.
2. Comments -This will permit your members to be able to comment and become involved in discussions on the material that you offer on each of your website pages.  This is perfect for smaller interaction, and it will definitely add value to your page, but it is harder to search for the comments that have been made.
3. Both Forum and Comments - If you would like to create more of an atmosphere of community then offering both a forum and the ability to leave comments is a great way to achieve this.
4. No Forum and no Comments – Generally the elimination of member interaction, occurs on the lowest end of products or the highest end of products.  For example, let’s say you took a small e-book and turned it into a membership site (low end product) then having comments or a forum is very much overkill.  On the other hand. if you sell a really high priced product, your message can get watered down too much with comments and forums, so you opt not to include.

For all options, except for forums, there’s not a lot you need to do. Everything you need is built into WordPress. You can add a comment plugin like Disgus. However, if you are going to include a forum then the complexity of your site goes up. Try to find a forum app that will work right out of the box to reduce the time necessary to learn. There are plenty of choices, but here are two options available for a forum.

Vanilla – This software takes a bit to get used to using, but once you figure it out, you’ll likely love it. It’s one of the best looking forum programs and there are numerous free plugins to interact with it and increase the usability. 

PhpBB – This is by far the most commonly used software for forums. It’s open source software so its free and it is an excellent choice because so many people are comfortable using it. In fact, the majority of forums online are built using phpBB. However, it doesn’t look as good as some others.