Apple Watch Activity Invites not Sending and Unable to Accept Invites

We've seen a handful of cases where the Apple Watch Activity app Fitness Sharing invitations aren't working; people are unable to send invites and unable to accept the invites sent to them by others. Here's what we've rounded up as solutions to this issue so far:

• If you are logged into iCloud with your @me.com email address instead of your @icloud.com email alias, try logging out of iCloud and logging back in with your @icloud.com email address.
• Try restarting both the Apple Watch and the iPhone, and then wait a little while. Then see if you can send invitations now.
• According to these two Apple Support forums [1] and [2], try un-pairing and re-pairing the Apple Watch. As you un-pair the watch, your iPhone will make a backup of the current watch state for you to restore when you re-pair.