To change the active group for a user thereby changing the group where the user is working in, navigate to Accounts in the Organisation Admin panel and open Manage Accounts. Find the current group using the email address. Open the actions menu of the group and choose Set as active group

Note that changing this setting while the user is logged in may affect the ability for the user to store or retrieve data in the group it was previously active in. 

Choose Set active group to change the active group for this user.