Addition is a valid, simple argument form in logic:

A.

Therefore, A or B.

or in logical operator (sequent) notation:

The argument form has one premise. From this two premis it can be logically concluded that A or B, must be true as well.

Here is an example of an argument that fits the form addition:

Democracy is the best system of government.

Therefore Democracy is the best system of government or everyone should vote.