You can now share signup links that have a pre-selected plan. This is especially handy when inviting people to sign up for a hidden or custom plan.

Find the plan that you want to invite your new member to under Manage Space > Monetization > Plans and click on the plan you want to share. Right under the name of the plan find the "Shareable registration link" and click on it to copy it to your clipboard.

Now all you have to do is to send this link to the person you want to invite to register for this plan. Aaaaand you're done.