I'm unclear on what the area of effect of Spirit Guardians is on a grid.
The spell has a range of
Self (15-foot radius).
Additionally, the description of the spell says that
You call forth spirits to protect you. They flit around you to a distance of 15 feet for the Duration.
The problem here is that on a grid, an "area of effect with a 15-foot radius" is not the same as a "all targets within a distance of 15 feet".
An area of effect with a 15-foot radius would fall under the general rules for areas of effect on a grid (DMG p.251), for example as discuss here.
The later could be interpreted as a range of 15 feet, or 3 squares away on a 5 feet grid.
On a 5' grid, what does the area look like?
Does it take one of the following shapes?
EDIT: this is not a duplicate of the Scroll of Protection question; the Scroll clearly has a cylinder area of effect, while the Spirit Guardians affects targets 15 feet away from the caster.