Well, as mentioned in this "Forest Groups
" post: "groups are supported, but for a different feature: to create clusters of objects that must be scattered together (e.g. clumps of grass and flowers). In this case, each item is handled as an independent item. You can see an example of use in the Forest 4 demo reel."
As mentioned in our on-line reference guide: "Do not use groups for objects that must be handled as a unique object (for example a tree and its leaves). In this case, attach all the individual parts into single mesh."
Actually, all objects which must be handled as an unique entity (as trees), all group objects have to be attached into a unique mesh.