User and Group Management

While working on Linux Essentials, I was experimenting with removing a user (me) from a group I created. Following online instructions, I ended up deleting myself from all the default groups and my only membership is now in my own group. Here are my questions: 1. How do I remove a user from a specific group? If usermod allows addition using -a -G, there must be a similarly simple way to remove the user from the group. 2. Is there a command that I can use to restore my group memberships? (i.e., sudo group, xdm group, etc.) 3. Finally, does manually editing /etc/passwd or /etc/group manipulate group memberships, additions, and deletions? As mentioned above, I just finished Linux Essentials, so I'm a beginner. Please keep that in mind when giving your input and any instructions that it might include.