How to fix "VM error: invalid opcode. invalid opcode"

This may not solve your immediate problem with Remix / EVM, but—looking at the code you shared—it seems you have a few lines inside of your for loop that ought to be outside, and executed conditionally:

for(uint i = 0; i <= uint(users.length); i++) { if(msg.sender == users[i]) { players[msg.sender] = players[msg.sender] + msg.value; } players[msg.sender] = msg.value; users.push(msg.sender); } 

Here, you are writing msg.value as the entry for players[msg.sender] once for every existing player in players[]; you then push msg.sender into users once for every player. This causes the following to happen:

1. The value of players[msg.sender] will always be msg.value, even in the scenario where msg.sender is already an entry in players (i.e., even when this particular player is already in the game). You effectively overwrite your "topping up" code in the preceding if block.

2. You end up with n+1 copies of msg.sender inside users, where n is the number of users that exist (in users[]) when join() was called.

I would step through each line of your join() code and verify that it's performing exactly the steps you expect. You may be able to avoid the for loop (which may be "unbounded"—see this definition) by modeling users as a mapping instead of as an array, and maintaining a separate usersCount state variable. This would let you update your players mapping in constant time—minimizing your gas costs.

You have the players[] array which is of type address and msg.value which is of type uint256. I can see a line of code players[msg.sender] = players[msg.sender] + msg.value; in your for loop whereby you concatenate different variable types. What I think is that you should create a new variable type of uint256 called balance and create a mapping of this new variable to each player in the players[] array. I am relatively new to this too so correct me if I am wrong