Penguins love to throw parties and igloos are a popular place to have them.
If you're a member, you can open up your igloo to other penguins on the igloo map.
Click the Igloo Viewer icon and select the igloo you'd like to open
Slide the button over to 'Everyone'
This will put your igloo on the igloo map and anyone will be able to visit
If you only want penguins you already know to visit your igloo, slide the button over to 'Friends'