Those students who have newly joined must have received a generated invitation mail yesterday, Monday, evening with a link to login to this site. If that mail is lost, you can request a new invitation by visiting https://edunet.lk/login/forgot_password.php and then type your e-mail in the lower box. Leave the Username box empty!
Then check your e-mail. There must be a mail to reset the account. If not, also look in the spam folder.
Once you are able to successfully log in, visit the IT Day home page, https://edunet.lk/course/view.php?id=440.
What you should do next is explained in the notice Students, select your topic for the IT Day! on the Notice board.
Note that everybody can request a new password as explained above. So any students who were invited some time back and can not still enter the site, may login using the above method.