Conformity bias and groupthink are very similar concepts, but they denote different phenomena.
Conformity bias is a broader term, and refers to people’s tendency to consciously or unconsciously change their behavior or opinion to fit in with others in a group.
On the other hand, groupthink refers specifically to a dysfunctional decision-making process where critical thinking and diversity are sacrificed because of a desire to maintain group cohesiveness at all costs. The result is premature consensus and a tendency to look down on those who don’t agree.
Conformity is one potential cause of groupthink. However, it can also arise due to the belief that dissent is impossible.