adding an ellipse to a PCA plot

Hi,

I created a principal component plot using the first two principal
components. I used the function princomp() to calculate the scores.
now, I would like to superimpose an ellipse representing the center
and the 95% confidence interval of a series of points in my plot (as
to illustrate the grouping of my samples).

I looked at the ellipse() function in the ellipse package but can't
get it to work. the princomp()-function gives me the scores of each
point, so I can calculate the mean and the 95%-CI, but I can't
integrate this into the ellipse()-function). Is there a better way of
doing this or can someone help me figure out this function?

best regards,

Lukas