It's easy to invite friends to your Magnet.
To add friends to your Magnet, tap on the "..." icon in the top right corner of the Magnet and select "Participants".
This will open a list with all your friends who you shared this Magnet with. Now tap on "Add People" below the list. Next, select how you want to share the invitation with your friends, and then send the message.
When your friends tap on the invitation and already have the app installed, it will automatically open the Magnets app and load the Magnet. That was it!