Track who is inviting new members to your server, give them a warm welcome with an image which also mentions who the person inviting them is.
You can customize the background and text colour, the following are some examples of this:
You can also enable Captcha requirements when they first join the server in order to get a role or just a simple reaction role.
Giveaways based on how many invites
Roles based on how many invites a user has