Activate the Whitelist functionality of Space Engineers
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The whitelist is a list that allows exclusive access to the server to those people added. The Whitelist role at Space Engineers requires users to first be added to a Steam group.
Find the group ID
You must first create a group on steam. Then write the following address in your browser, replacing the name of your group:
https://steamcommunity.com/groups/(NOMBRE DEL GRUPO)/memberslistxml/?xml=1
The group ID is on the second line
Adding the group to the whitelist
Once the members are in the group add the Group ID 64 to the whitelist field.
Those people who are not in the steam group will not be able to enter the server.