To find out how much of the pan of brownies Marcel and Bryan ate together, you can add the fractions of the pan they each consumed.
Marcel eats \( \frac{3}{8} \) of the pan, and Bryan eats \( \frac{2}{8} \) of the pan.
Now, add the two fractions:
\[ \frac{3}{8} + \frac{2}{8} = \frac{3 + 2}{8} = \frac{5}{8} \]
So, together they ate \( \frac{5}{8} \) of the pan of brownies.