As of a recent update you can now create passworded Usergroups,
By default these groups are 'Admin' and 'guest'.

Here is an extract of a default starbound_server.config:

 "serverUsers" : {

      "Admin" : {

          "admin" : true,

          "password" : "passwordhere"

      },

      "guest" : {

          "admin" : false,

          "password" : "passwordhere"

      }

You can see that these groups can determine whether the users have access to admin commands.

If this is set to true, you must also make sure to set this line within starbound.config:
  "allowAdminCommands" : true,

If you were to add another group, say the group is names 'livestreamer' here is an example below:

 "serverUsers" : {

      "Admin" : {

          "admin" : true,

          "password" : "passwordhere"

      },

      "guest" : {

          "admin" : false,

          "password" : "passwordhere"

      },

      "livestreamer" : {

          "admin" : false,

          "password" : "imapassword"

      }

You would then login to Starbound using the details:

ServerIP: yourip

Username: livestreamer

Password: imapassword


If you require information on forcing login in with a user account, check the guide 
Why can anyone login without a password?