Can colleagues watch a course together on one screen?

You can, but only the person who's signed in gets the certificate. Certificates are personal, so everyone needs to take the course on their own account to get one.

If you've used a course for a group session, such as a staff meeting or INSET, a school administrator can record it for everyone who was there. Go to Manage, then Records, and press "Add training from elsewhere". Choose the group, add the date and hours, and attach a register if you have one. It then shows in each person's training record, though it isn't a Creative Education certificate.