Definitely, students can join sessions without having to sign up on MeritHub. This can easily be done by joining through a participant's link provided by the host. The participant simply needs to enter his name (and email) before entering the classroom. Thus, students even without an account on MeritHub can join a session.
When a new session is being organized, under the Settings section, there is a checkbox that is marked by default implying that the participants can enter the classroom without signing up.