• Make sure the person you wish to remove has no outstanding debts to anyone in the group. (If you have Simplify Debts on, they only need to have a $0 balance; if you have Simplify Debts off, they need to have all individual debts repaid.)
• Navigate to the group settings page by tapping the settings icon (in the upper right corner of the group's screen in your mobile 
• Tap on the person's name that you'd like to remove
• Select the option to remove them from the group

And...you're done! :)