When you are invited to join a team or you are invited to an event, you will receive a notification. You can view your notifications and accept or decline events right from the notifications page. Get to this page by clicking the envelope icon in the top right corner of the screen. In the dropdown menu, click 'notifications'.
If you are on TeamPages classic, you can click 'my notifications' in the menu bar.
Your notifications area will include things like invitations to events, and you can mark your attendance as attending or not attending. This is how your managers and coaches know if you are going to be at events, so make sure to check your notifications regularly.