1. Putting yourself or someone else on a list for an activity, class, etc.
2. When someone is instant messaging you and they suddenly sign off because of something you say (similar to "hang up" on the phone)
guy1: i heard this year mr. herbert is teaching woodshop.
guy2: i have to go sign up for that class!
girl1: what happened last night? you totally signed up on me!
guy3: sorry, my internet connection is horrible.