A special emigration office which deals with emigrations to the Moon, to Mars, etc. The USA was the first to establish an ETE office, in 2036, named American Space Travels [AST]. At that time space emigration was limited to the Moon.

Later, in 2039 when Areopolis was founded, Mars was open to colonists so AST put the Mars Colonization Plan into practice [part of the Mars Act from 2023]. This described how to begin the Mars colonization project succesfully. In short the concepts of an ETE Office, a selection procedure, etc were defined.

In Europe time didn't stand still. The Mars Colonization Plan from the USA was adopted and turned into the European Mars Colonization Plan. This resulted in the foundation of Earth Mars Travel, the European Mars ETE Office, with it's prized European Selection Procedure: a completely watertight selection programme.

Emigration to the Moon is a different story. The Moon is so close to Earth that a number of companies have designed spacecraft which can bring emigrants to the Moon. This however is very expensive. A cheaper way is to book the extra space in the Mars Shuttle used for the Mars emigrants.

Considering the Asteroid Belt colonization this is totally arranged and maintained by the ISA.