Can our private coach drive here?

I would think the charter coach company would be in the best position to answer that.

How big is the bus? Buses higher than 4mt and longer than 10.36 mt are prohibited from accessing the Amalfi coast during most of the year. Also, during certain critical days (holidays etc.) traffic restrictions apply to all buses of any size.
From 7:30 to 24:00 hours, buses shorter than 10.36mt and lower than 4mt are permitted (except for those critical days above) but only in the direction Positano ----> Vietri sul mare.

Estelle, If you are able to get the Coach to Positano using the directions that Roberto provided, you'll have to park on a promontory above the town and transfer your group to smaller vans to get into the town. I'm sure the Coach firm is used to the routine there, so they'll have that arranged. Happy travels!

When I was on an RS trip to Italy - our bus could not go into Positano.
It stopped out on the road on the way going south. We were all put in Taxis to our hotel in Positano.

Thanks everybody! I'll consult with my driver beforehand. Perhaps just easier to take public transport from Sorrento to Positano and back for this excursion. We shall be in Capri for the day. Wondering now whether we should perhaps transfer from Capri to Positano by sea (ferry or hydrofoil) and then take a land transfer to return to our accommodation in Sorrento later the evening ???
WHAT is the best of the 2 options?

Definitely not easier to take public transport to Positano. You may well find that not all of your group will get on the bus or that some people will have to stand! They are very crowded. BTW - last week a tour organiser with a group of people, who had been waiting in the long lines at Sorrento, organised a private bus for her group on the spot! Neither will the bus go down into Positano ie you have to walk down so no difference there. The last part of the journey down to sea level is a pedestrian area. The journey back to Sorrento can be even worse because the bus often fills up in Amalfi. I have been on the bus when it has just driven past people waiting to get on at Positano and other stops along the way.

OK - so I take it that I should rather get a private transfer organised if my OWN bus is too large to travel the road.
Thanks All