Minimally and sparingly. One person can do this a couple times a week without causing permanent harm.  However, you must avoid walking in a single path, standing in one place, or trampling your plants.
If you must service a wall, parapet, or rooftop equipment, wait until the foliage is dry and conditions are not too sunny or hot.  Then, cover the plants with plywood or closed-cell foam for up to 4 hours.
Avoid repetitive foot traffic when plants are wet or frozen.
Whenever possible, avoid installing the green roof until all other trades have completed work on the skylights, parapets, HVAC or other rooftop equipment.  In our experience, the most common cause of plant losses are foot traffic from other trades performing work to equipment adjacent to the green roofs.