Group bom is a bom maintain in engg phase(R&D) it maintain
without plant assignment function.
Bom group is maintain for all kind of same material bom
which has same characteristics and classes eg.all RM /SF/ F
bom

Group BOM:- Group BOM is a type of B.O.M like simple B.O.M, 
Variant B.O.M, Multiple B.O.M.
It is made without reference of any plant after creating 
it, we can assign it any number of plants but in this case 
internal number of B.O.M will remain same & also any change 
to main bom will effect every bom where they are assigned, 
but if we will make any b.o.m from this group B.O.M by copy 
through than internal number will be change & in this case 
change in main bom will not effect other bom.

B.O.M Group:- When we create a B.O.M a bom group is given

A bill of material that is not assigned to a plant.

A group BOM is valid across the entire company.

Special features of group BOMS:

A plant check is not carried out for the materials in a 
group BOM. 
A group BOM is not exploded in material requirements 
planning and in the production order. 
Group bills of material are mainly created in the design 
phase and then later assigned to one or more plants for 
production.

A collection of bills of material that lets you describe a 
product or a number of similar products. 

The following bills of material comprise a BOM group:

All the variants of a variant BOM 
All the alternatives of a multiple BOM