Request random people without an explanation: When you actually know the person you’re requesting, you can jog their memory by marking a box showing how you know them. When it’s a stranger, they’re going to wonder what exactly you want. Think about how you react when someone you don’t know on Facebook requests you? Ignore! You don’t want that to happen here. If it’s someone whose career you admire or you’re inquiring about job opportunities just drop them a quick note to say so.
Upgrade: Unless you’re about to go hard. With a basic LinkedIn profile there’s enough free information to get what you need. The site is just a connector between professionals what you really want is in-person contact or even personal emails. For most people, it doesn’t require a $20/month or more commitment to do that.