It’s a business networking site, a bit like Facebook but less intrusive and more professionally oriented.
The invitations to “connect” can be a pain sometimes. It’s helpful if you are looking for work or trying to change careers.
I’m on it, but don’t really pay much attention to it. Don’t know why your friend had invitations sent out on her behalf if she was not aware of it. It’s possible that your friend’s e-mail may have been hijacked or she checked a box somewhere without realizing it, but the actual site itself is not a scam or malware.
Linked In has started a “feature” that links your address book contacts. I won’t turn it on because I don’t want them contacting people without my knowledge.