LOVOO invitation codes are special voucher codes in the form "xxx-xxxx-xxx" that can be assigned in certain special offers and only redeemed on the LOVOO app.
Open your profile settings to redeem an invitation code. You will find the item "Redeem voucher code" at the top. Enter your personal code there and click on "Check" and then "Redeem now". Please note that it may be necessary to restart your app so that all changes can take effect correctly.