No visa is required for EU nationals to enter the Federal Republic of Germany.
A valid ID card or passport is sufficient.
Nationals of all other countries must obtain a visa.
For semi-annual visits of up to three months, a visa is not required for those countries for with the EU has abolished the visa requirement.
Detailed visa information, including a list of those countries for which a visa-waiver agreement exists, is provided by the Federal Foreign Office ("Auswärtiges Amt").
If you require an invitation letter for the visa application, contact the conference secretary.