The sentence plan is incomplete, but apart from that there is an error at the beginning of the sentence:
Should be: "The Grand Duke of Hessen invited Behrens to stay in Darmstadt in 1899 and to join a group of young artists"
Is: "The Grand Duke of Hessen invited a stay in Darmstadt and a join to Behrens ."
The sentence plan is incomplete, but apart from that there is an error at the beginning of the sentence:
Should be: "The Grand Duke of Hessen invited Behrens to stay in Darmstadt in 1899 and to join a group of young artists"
Is: "The Grand Duke of Hessen invited a stay in Darmstadt and a join to Behrens ."