From Cindy Bailey:

 Heres is a brief video that I recorded (approx. 3 minutes) that shows a few ways that you can encourage your shy students to use Flipgrid.  I have a student in one of my classes who hates seeing himself on camera and I get the feeling he simply won't complete my Flipgrid assignments (of which there will likely be several during this remote learning process), so I played around and found a few options that students can use to still be in the video but "cover" their face.  

I hope you will find it useful.  Feel free to share it directly with your students if you want...that might be easier than trying to explain it to them in writing, although as long as you have the right settings applied on your grid topics, they usually find the right buttons on their own to play around with these features.