A Guardian Demon, formally known as a Protector of you, is the exact same thing in-definition of a "Guardian Angel". However they're followers of their Master Satan instead of God (Yahweh/Jehovah) or Jesus Christ the Lord. The concept was created by Joy of Satanist member high-priestess Maxine in response to a friend who she was working on a Ouija Board with. So they asked if there was a Guardian Demon for her, and the planchette spelled out G-U-S-I-O-N.
"Later on, Gusion appeared to her in a dream. A couple of weeks later, I drew his sigil, we took it to a tattoo shop, and she had it tattooed on her arm. She has made some major improvements and things have been going well for her." --Quote by: High-Priestess Maxine Dietrich. 
These demons have an interest and do want to make contact us, and are happy to contact you.
Their Job: Edit
Before the individual becomes involved or aware of the works for their Guardian Demons his or her main job is to bring the person closer to Satan. Often learning of the individual during these tasks, for example how to mimic him or her often this comes in handy when a Medium or other person similar to that in expertise tries to contact the deceased individual when they die. Through not only do Guardian Demons pose as the person a Medium or the person trying to contact the deceased but many demons do this as well, the demon will try to pose as the individual.
There job according to Maxine's Satanism is to be a protector to us. Lead us closer to Satan, guiding us, comforting us, and if we're good in our ways of Satanism bring us gifts. There true job is to inform Satan of us and lie to us, they're great liars and are an example of the very friendly angel guise.
They'll help you with the Satanic religion and how to better relate to Satan when you're new to the occult, they're a recommended demon to summon. If you use a Gurdian Demon
These demons definitely want you to know of them and contact them, and are happy to come into contact with you.
Contacting them: Edit
However, like many demons they can be dangerous if a person is unprepared for the outcome and isn't full of common sense or reason. Of course you can always state that you're allied with Satan (and do not lie to demons for the conseqences can be devastating). Summoning these demons is a lot safer than other demons. However, if you use them for malintent they can be very dangerous to the summoner. They can be helpful through still harmful if not careful
Read the link bellow yourself, I'm not responsible for a lost soul or soul death. But please don't do rituals on the site or be involed, I'll pray for you. Please understand that this is your last warning, Jesus exists and is Lord. 2 (Thessalonians 1:8)
No Real Guardian Angels, So Why Create a Concept of Guardian Demons: Edit
Despite the facination with a "Guardian Angel" figure in the Bible there appears to be no Guardian Angel that truly exists but are known as Protectors to you.
"There is no such thing as a guardian angel. Some ancestors who have deep affinity for the family communicate to help their family members regarding worldly matters. These departed ancestors may be mistaken for the guardian angels. Ancestors within the range of spiritual level between 20-30% can help only about deriving worldly pleasures. Only those ancestors above the spiritual level of 50% can help regarding spiritual progress." 
 Angel information from an organization who studies them.