I'm a resident in Germany and I live in a WG room (‘flat share’). My girlfriend is applying for a visitor visa to come to Germany. I'll be giving her an informal invitation letter with my required documents.
I want to know if there is any kind of official letter (which I can get from Rathaus) necessary for hosting a person?
As I live in a WG room and she will be obviously staying with me, I want to know that is there any requirement of the area of the room for two people to live in??
There is nothing you need to get to host someone. If she stays for more than 6 weeks your landlord might have a say, but otherwise not.
In practice, the most important thing will be proof of health insurance that covers any eventualities during her stay, because German health insurance is usually either connected to employment contracts, shared inside a family or taken out privately for longer terms, neither of which apply here.
There is the infamous "Verpflichtungserklärung für einen kurzen Aufenthalt" that replaces proof of health insurance by effectively making you a guarantor, but it is doubtful that would be accepted unless you can prove sufficient income or funds, and it'd be expected that the funds are locked for the validity period (six months), so if you can show any other proof of health insurance for her, that would likely be preferable.
From an immigration point of view, the only thing that matters is that there is a name and an address where people are likely to be reachable, which is the same level as for residents -- local conditions at that address are irrelevant.
