When you receive a VR Call, click the link in your email and select that you want to open the VR Call link with the AltspaceVR app. If you don't have the AltspaceVR app installed, you'll need to download it for free from the Oculus Store. You'll be directed to the store to download it when you click the link.
Once you have all the necessary software installed, just click the "ENTER" buttons on the VR Call to enter directly with Gear VR. You'll be prompted to insert your mobile device in the Gear VR. You'll arrive in VR at the Space designated in the VR Call. All other invitees will also arrive in this same Space.