Space Engineers utilizes a steam group system to keep your server private. To set it up just follow the instructions below
1. Create a Steam Group and set the 'Custom URL'.
2. When you have created your steam group go to
http://steamcommunity.com/groups/[GROUP NAME HERE]/memberslistxml/?xml=1
(Making sure to replace GROUP NAME HERE and the  brackets with your actual group name)
3. Copy the long number within the <groupID64> tags.
Then log into your game panel → game services → select your server → configuration files → configuration editor
4. Paste your group ID in the field "Steam Group ID"
Your server should now only appear to those in your steam group.
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