The ornamental design for a portable groundingmatedge, as shown and described.
FIG. 1 is a perspective view from above of a portable grounding mat edge having two perimeter stripes according to my design;
FIG. 2 is a plan view from above of FIG. 1;
FIG. 3 is a plan view from below of FIG. 1;
FIG. 4 is an end view taken from the left side of FIG. 2;
FIG. 5 is an end view taken from the top end of FIG. 2;
FIG. 6 is an end view taken from the bottom end of FIG. 2; and,
FIG. 7 is an end view taken from the right side of FIG. 2.
The broken lines of other features of the mat onto which my design is incorporated show environment for illustrative purposes only and form no part of the claimed design.