If you accidentally decline a request you can always add the player back to the club manually. To do this follow this user guide: How do I manually register a player?
Once complete best practice would be to email the player informing them of the addition, as well as clicking the "Send Claim Invite" button found under player options(this is the drop down menu found on each player listed on your club). This will allow the player to take control of their profile and update their contact information for you.