There are 3 groups. These include:
Participants (do they know which group they are in?)
Researcher / clinician delivering the intervention (do they know which group they are delivering the intervention to?)
Researcher / Assessor measuring Pre and Post intervention (do they know which group they are measuring?)
NB Simply Single is where any one of these is blinded, double is where any two and triple is all three.
Further Reading: This article explain the complexity of understanding blinding.