There are two ways in which you can join a video call that happens in a Message session that you participate in.
Option #1:
You will automatically be prompted to enter the video call that is initiated by another participant. On the 'Video Call Request' screen, if it is not already done, enable your microphone and camera, then click 'Accept'.
You will be taken directly into the video call.
Option #2:
You will see a banner at the top of the chat screen, alerting you that a video call is in progress. You may tap on it and then you will see a 'Join Video Call' screen. If it is not already done, enable your camera and microphone, then tap on the 'Join Video Call' button.
You will be taken directly into the video call.