In Norwegian that would correspond to: "Bli med i Erasmus".
One might aslo say, as Bicontinental's suggestion in Dansih: "Vær med Erasmus". However in Norwegian, there is a distinction between "bli" and "være". "Bli" expresses a change, in this case if you are not in Erasmus, join. Være express a conitunation, in this case, if you are already involved in Erasmus, keep on. I became aware of this when I learnt German in school, they have the same with "bleiben" and "sein". Therefore I would assume this is the case in Danish too? How about Swedish?