Conclusion: there is nothing wrong with you joining the sports club as mentioned in the question, and what you have mentioned about matters concerning which there are reservations can be avoided easily, praise be to Allah. And Allah knows best.

To sum up all of the above: prostration to anyone other than Allah does not constitute disbelief unless it is done by way of worship. But if it is done by way of greeting, it is a major sin but it is not disbelief.