George van Eps (1913-1998) is often called the father of the 7 string guitar. His 7 string guitar had an extra low string tuned to A or B and was not mended to be strummed like our hard rock heavy metal friends do on their 7 strings.
George was a bit more sophisticated and used the extra bass string to give his playing a bottom like a bass player was accompanying him. George was an example for the next generation players like Howard Alden and the family Pizzarelli.
In the United States the 7 string jazz guitar is very popular and many luthiers like Benedetto and Unger (see picture) produces a lot of 7 string guitars. Here, in Europe, they are pretty rare.
I once built one for Tony Marshall (UK , France) and here in the Netherlands Peter van Weerdenburg is one of the big promotors of the 7 string.
The seven-string guitar adds one additional string, commonly used to extend the bass range (usually a low B) but it can also be used to extend the treble range of the 6 string guitar.
The additional string is added in one of two different ways: by increasing the width of the fingerboard such that the additional string may be fretted by the left hand; or, by leaving the fingerboard unchanged and adding a "floating" bass string. In the latter case, the extra bass string lies next to the existing bass strings, but free of the fingerboard in similar fashion as the archlute and theorbo. Such unfrettable bass strings were historically known as diapasons or bourdons.
List of artists who use seven-string guitars