To change group access for current members: from your Dashboard, select Members and then search for the member you'd like to modify.
Select the members name and scroll down to the Group Memberships section at the bottom of the page. Start typing the group name you'd like to add the member to and select it from the dropdown. Click Share Access.
From the new window that pops up, type in the group name again and select if from the dropdown menu, then click Save.
The new group will now be added to the member's profile. If you wish to remove the member from the previous group, simply click the little trash can icon next to the group you wish to remove.
You can also follow the steps in this video.