Once you have shared your invite link or code, here is what your partner needs to do.
Tap the invite link you shared. This will direct them to download Cupla if they don't have it already, then straight into the connection flow.
If using a code, they can enter it on their Cupla homescreen once the app is installed.
Create their account and complete the onboarding — see How do I set up my account?
Once connected, it may take a minute or two for everything to sync. If you have an active subscription, your partner will not be shown the subscription screen — see How does my subscription work?
Your partner already having a Cupla account is not a problem. They can accept your invite from the account they are signed in to — they do not need to log out or start again. The invite is only refused when they are already connected to someone, and that includes being connected to another account of yours. If the link or the code keeps failing, see We look connected on my partner's app, but not on mine.
If you are having trouble connecting, see How do I sync with my partner? or message our support team from the Cupla app.