Databse design for a part/subassembly list
To reproduce this structure, you need an Assembly table referencing itself through a ParentAssemblyID and a Part table. The diagram in Access looks like this:



Note that the assembly table appears twice in this diagram. But there's really only one unique table named Assembly in the DB.

AssemblyID and PartID must be declared as primary key (PK). Its easiest to declare them as AutoNumber column.

In the Realtionships diagram, insert the Assembly table twice, so that you can draw a relation between them. Right click the relation lines and check the boxes "Enforce Referential Integrity" and "Cascade Delete Related Records".

Your example data would have to be entered like this

Assembly
-----------------------------------------------------
AssemblyID  Assembly                 ParentAssemblyID
----------  -----------------------  ----------------
  1         Main Assembly
  2         Group 1: SubAssembly        1
  3         Group 2: SubAssembly        1
  4         Group 1: SubSubAssembly     2
-----------------------------------------------------


Part
---------------------------------
PartID  AssemblyID  Part
------  ----------  -------------
  1       1         Part 1: Rivet
  2       1         Part 2: Rivet
  3       2         Part 1: Rivet
  4       2         Part 2: Bolt
  5       2         Part 3: Bolt
  6       4         Rivet
  7       4         Rivet
  8       3         Rivet
  9       3         Bolt
---------------------------------




Answer to follow-up question: "How would I structure it if I wanted the same part to be usable in many assemblies?"