Anyone know a 'count me in' plugin?

Hello.

So, I’ve built a website for a clan that plays Battlefield. I’d like to add a page with a picture that states an announcement for a clan match. I already use member groups and such, but I’d like to add some kind of ‘count me in’ plugin that a member needs to click on, to confirm their presence.

Does anyone know if that kind of plugin exists?

Thanks in advance.
Best regards,

Natalie