It's okay to make a new post with "Join 14:00" in the subject line when you're doing an official sprint. I think replying to mini-sprint posts are important so other people doing minis will know they aren't alone––since a mini-sprint doesn't "count" unless there's more than one person sprinting.

That's how I understand it anyway. As long as you post your join, you're all set!

Cheers!