Even though WordPress Multi-site allows you to have multiple blogs or sites running from one WordPress install user account access is dealt with differently in multi-site vs a single WordPress install. In a standard single install when a user is added to the site they are usually given the “subscriber” role. This role allows them to sign into the site as a user. With multi-site all the sites or blogs within the WordPress Multi-site share a single user database.
So if a user account is added to the WordPress Multi-site one would think that they have access to all the WordPress sites within the Multi-site network. Well that is not the case. Each site is treated as a separate entity even though the user account is added to the network. The user account that you created has to be given access to the specific site that you want. Now you might think well that does not seem to advantages? that is the farthest from the truth. With this method lets say that you added the user on “Site A” as a subscriber but on “Site B” you want that same user to be a “Contributor or editor” that is possible.
The advantage to this is that you can assign one specific user different roles based on the site that they need access to. Lets take a real world example:
I have a blog site that I created on my WordPress Multi-site and a friend either signs up for an account on my site or I create them an account. Now my friend likes my blog and wants to start their own blog. I create a blog site for them within my WordPress Multi-site. Even though my friend is a “Subscriber” on my site I can assign them as an “Administrator” account within their own blog site. So now my friends account is assigned two separate roles for two different sites with only one simple single sign on.
This type of architecture allows you delegate user access levels per site. The only user account that has all across the board in a WordPress Multi-site install is the “super” account. This account is the network administrator for all site. This account is able to (add, delete and install plugins), (add, delete and install themes), manage all accounts across the board, (add, delete and modify pages) across the board.
The “super” account or better know as the “Network Administrator” should not be given out lightly as it can cause a lot of damage if use incorrectly. One thing to keep in mind in WordPress Multi-site if you bring one site down then the chances are you brought all the sites down.