planning tool – web "social" calendar

I am developing a site for a group that wants to get together and play games and grill out and what not.

Is there some kind of free app out there, or simple code, that can be implemented that acts like a social event calendar where user can register and then RSVP to events and email and text message reminders can be sent (text through the @vtext.com option -Verizon for this example)

Thanks

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