Participants should secure accommodation by themselves. The internet booking services as well as Airbnb have good selection for the Helsinki region
Google Hotel Search:

Scandic Hotels offers 10 % discount with the booking code FIARB. Click “Booking code, Bonus Cheque or Reward night” and add the code FIARB to the textbox “Add booking code”.

Some recommended hotels nearby the meeting venue:

Original Sokos Hotel Torni  

Radisson Blu Plaza

Original Sokos Hotel Vaakuna

Hotel Marski by Scandic

Hotel Seurahuone

GLO Hotel Kluuvi

Hotel Anna

Klaus K Hotel

Hotel Haven

Hotel Lilla Roberts