"Has become" or "has gotten" are interchangeable. I would suggest: We've gotten stronger since you became a part of the team. Our team is stronger since you've joined.

Our team will be stronger with you. Our team is stronger now that you’re on it. ‘Has become’ and ‘has gotten’ are both wrong because it talks of a changed present result. But that hasn’t happened yet. And ‘gotten stronger’ isn’t appropriate in a business setting. This use of ‘Get’ is too informal.