sign in/up

[Lenka]

What is the difference between the phrasal verbs "sign up" and "sign in". (Is there a difference (in the usage) at all?)

[Barb_D]

When you "sign up" for something, you show your interest in participating.
Let's sign up for the volleyball team. Jim signed up for photography lessons.

When you "sign in," you are making it known that you are now there. You can sign in to your computer (like logging in) or you can sign in when you arrive at class on the attendance sheet.

[Lenka]

When you "sign up" for something, you show your interest in participating.
Let's sign up for the volleyball team. Jim signed up for photography lessons.

When you "sign in," you are making it known that you are now there. You can sign in to your computer (like logging in) or you can sign in when you arrive at class on the attendance sheet.

Thank you very much for the explanation!