The demons attack the nearest non-demons to the best of their ability. As part of casting the spell, you can scribe a circle on the ground with the blood used as a material component. The circle is large enough to encompass your space. The summoned demons cannot cross the circle or target anyone in it while the spell lasts.
Is the creation of the circle magical in nature? Could the party Fighter, in order to protect him/herself, create a circle of blood that would ward off the demons?
As part of casting the spell, you can scribe a circle on the ground with the blood used as a material component.
Because it must be done as part of the casting action and with the same blood being used as the material component for the ritual. Since it goes on further to say "your space" I believe that its fairly clear it can only be used by the caster to protect themselves during the casting of the spell.