You probably saw money being transferred from a player to another on the chat, here is how:
You can write the following command on the chat, and a window will open: /tip <username>
Example: /tip gustavo
Now all you have to do is choose the amount you want to send, select a coin, put your 2FA (in case you set it) and press "Send Tip". Easy!
You can also click on any nickname on the chat and click on the BTC symbol to send a tip, same previous window will open.