Yes! You just need to launch the Activity App (or the Fitness app if you’re on iOS 14 or any advanced one) and then touch on the Sharing tab, which is located in the bottom right-hand in the corner of the screen.
After tapping on the plus sign, enter the contact information for the friends whose activities you want to share with them.
To send the message, tap the Send button in the top right corner.
Your friend will get a message informing them that you would want to share your activity with them, and it will be up to them to decide whether or not to accept the invitation.