[I'm not sure if helping students with English classes is off-topic (but I can't find mention of it). But given how old the question is:]
I believe the exercise is looking for the present perfect, because the previous sentence is in the present perfect. Given that your class is all about the present perfect, we can also assume that that is what the exercise is looking for.
I'm happy because many good players have joined the team.
The present perfect is formed by using has/have + past participle. "Many good players" is plural, so you need the plural 'have'.
In theory, the present tense also makes perfectly good sense in this context
I'm happy because many good players are joining the team.
But given that the speaker says 'last month' the team bought a new player, one might lean towards expecting the present perfect. But either sentence is perfectly acceptable in the context given.