Here is an outline for odd order $p$-groups:

The main point is that every non-zero element in a field with at least three elements is a sum of two non-zero elements. Using this, you can show that irreducible matrix in $GL_n(\mathbf Z/p\mathbf Z)$ can be written as a sum of two non-singular matrices, for such an irreducible matrix corresponds to multiplication by a primitive element of the field of order $p^n$.

Now, using the Jordan canonical form matrices over a finite field (see Theorem A.22 in [my notes][1]) you can show that every non-singular matrix over $\mathbf Z/p\mathbf Z$ is a sum of two non-singular matrices.

Finally, the endomorphism algebra of a finite abelian $p$-group modulo is radical is a product of matrix groups over $\mathbf Z/p\mathbf Z$ (see section 6 of [this paper][2]). So any invertible element of the endomorphism algebra can be written as a sum of two invertible elements modulo the radical of this ring. But adding something in the radical does not affect invertibility.


  [1]: http://www.imsc.res.in/~amri/html_notes/notesap1.html#x8-32000A.4
  [2]: http://arxiv.org/abs/math/0611897