Hello there!
Have you contacted the Armenian embassy to ask them precisely about the invitation letter? It's very different for each country, you should ask them thoroughly to know who is eligible to send you an invitation letter (for example some countries only let the natives who own a well-paid business and property send invitations - it's really different from country to country, thus you definitely should ask the Armenian embassy in the country you live in at the moment in detail).
It also depends what kind of visa you're applying for. If you're applying for work visa you can get the invitation from the company you'll be working for, if for studies - you can obtain the visa by easily applying for the university and obtaining your acceptance letter, but it's definitely different for Tourist Visa - there'll be thousands of different ways to obtain a Tourist Visa, one is for example travelling using Tour Agencies.
