datitisev electrolyte There's another table called users_groups which has a user_id and group_id. Find the matching user_id to the user in question, and the group_id listed will be there. The table supports multiple records per user_id (for multiple groups), which means if you want to remove someone you'll need to find all their records. You can do this by using the following query:
SELECT user_id, group_id
FROM users_groups
WHERE user_id = 1
Make sure to prepend users_groups with any database prefix you entered during installation (mine is flarum_ for example) and change the number at the end to the user id in question.
Consequently, if you want to see all the users in a group, you can do something similar:
SELECT user_id, group_id
FROM users_groups
WHERE group_id = 1
This will show you all administrators (group #1) on your Flarum install. Change as needed.