Letters of invitation can be sent by the conference office on request. These do not imply acceptance of abstracts or funding by the organisers of the conference.
The visa regulations can be found at the website of the Federal Foreign Office.
EU nationals do not require a visa to enter the Federal Republic of Germany.
Generally speaking, all other foreigners require a visa for stays in Germany. A visa is not required for semi-annual visits of up to 90 days for nationals of those countries for which the European Community has abolished the visa requirement.
You will find an overview on visa requirements here: Table of countries whose citizens require/do not require visas to enter German