add --groups [groupname] flag (#1006)

Author: ethanpalm

installation.mdx

@@ -86,6 +86,16 @@ If you attempt to run on a port that is already in use, it will use the next ava
 Port 3000 is already in use. Trying 3001 instead.
 ```
 
+## Previewing as a specific group
+
+If you use partial authentication to restrict access to your documentation, you can preview as a specific authentication group by using the `--group [groupname]` flag.
+
+For example, if you have a group named `admin`, you can preview as a member of that group with the command:
+
+```bash
+mint dev --group admin
+```
+
 ## Additional commands
 
 While `mint dev` is the most commonly used command, there are other commands you can use to manage your documentation.