Check that the email address has no typing errors or check to see if the e-mail ended up in your Spam folder. Network spam filters like those sold by Barracuda could also be blocking the e-mail invitations.
This is caused by lack of vídeo codecs on that PC, please update them. You can download a complete video codec pack here: https://files3.codecguide.com/K-Lite_Codec_Pack_1544_Full.exe
This user has previously been registered as a Collaborate Space user, hence their password is needed to connect. They can reset their password here: https://www.collaboratespace.net/#/forgot