Above the contact list and search box there is a contact list filter, which says "All". If you click on this and select the group you want to see, then you will see all members of that group.
By clicking on the checkbox to the left of the members of the group, you will see the collection of contacts you want to take action on, listed on the right.
When you have finished selecting the ones you want to remove from the group, click on the Group menu option above and uncheck the groups you want to remove these people from.
If you wish, you can even add them to a different group at the same time.
When you have finished checking or unchecking the groups listed, then click on Apply. This will update the chosen contacts as appropriate, and given a few minutes, the next time you view the group membership it will contain only those you kept.