Cox uploaded a short video of herself with Aniston whispering something in her ear as Kudrow looked on.
"Friends don't let Friends skip elections," the video says at the bottom.
"Text FRIENDS to 26797 to make sure you are registered. And tag your FRIENDS below to remind them to check their registration #RegisterAFriendDay @imavoter," Cox captioned the post.
Kudrow uploaded a photo of herself on Instagram posing with Cox and Aniston from the video. The actress also supported I Am A Voter's campaign to register more voters. I Am A Voter is a nonpartisan movement aimed at increasing voter turnout.
The reunion special was postponed in April due to the COVID-19 pandemic. Schwimmer recently said that the cast is hoping to shoot the special in the middle of August if it's safe.