Currently, one person cannot be in more than one group at a time. You will have to create a new "friend" in the second group, and record a new video for that friend.
Or, to make this easier, we recommend recording the friend's video in the Camera app on your iPhone or iPad (instead of the Friendclips app itself) so it is saved to your camera roll. This way, you can use the same video for the same "friend" across different groups.