I doubt you'll find a group you can join from Helsinki. But DIYing is simple. Buy a train ticket at www.vr.fi and hop on the train. The long haul busess stop by the Turku cathedral and that's a great place to start wandering too. Tickets and timetables at: www.matkahuolto.fi
The visitturku.fi/en/…tourist-information can help you build an itenerary for whatever season youre coming or set you up with a guide. It's a pretty much walkable city too if you haven't any special needs.Edited: 17 September 2017, 11:51
The page above has an online chat possibility at Finninsh office hours 9-16 GMT +2. Closest thing to private free professional planning service you can get.
(TA DEs are amateurs...)
Thank you; I thought about same - I'l reach out to build an itinerary. If you have any personal favorites in Turku, please share. Thanks!Edited: 17 September 2017, 20:16
Walking along the river, the market hall, the restaurant scene in general, Aboa Vetus.
The riverside stroll from the cathedral to the castle covers the main points of interest. Also check out the indoor market.