"They intend to join the team."
Translation:Ili intencas aliĝi al la teamo.
On the one hand, it seems you are right in the sense that Tekstaro finds only one case of "aliĝi" + accusative ("Kiam iu aliĝas organizon, li do jam devas scii, ke"); on the other hand, I have heard it said that most times in Esperanto, one is allowed to replace an object of a verb in the form of preposition + noun with the accusative form of that noun.
It might be that this form is just unusual in this particular context, with "al" being much more common, but not quite wrong? I'm not sure; just speculating.