We have recently made a number of changes on how the members of an eForm are added. It now is a smart group based on the groups which have been dragged into the group. Before these changes, groups were comprised of individuals from the groups.
You may be trying to add a group to an eForm which was made before this change.
To work around this, just click into the group you are trying to add, select all the members and then drag them into the eForm. This works exactly how dragging the group worked previously.