To attach all grouped meshes into one object you can use Attach command under Editable Poly / Mesh. First you need to Ungroup it (Group - Ungroup). Then select any of meshes and check under Polygon sub-level it's Material ID (you can set to 1, second object to 2 and so on). For each geometry you should create slot in Multi/Sub-Object material with corresponding ID number. Be careful to treat all different materials under corresponding IDs to avoid material mismatching while using Multi/Sub-Object material.
Anyway, in recent releases of Forest Pack also group can be assigned as a Custom Object (note: select the group by name, not picking in the scene). Elements of the group are scattered together, but Forest Pack handling them as individual items.
Hope this helps you, but in case of any further question or doubt, please don't hesitate to contact us. Have a nice day.