For the longest time, I would dump any contacts email address into LinkedIn with an appendage to the first name to remind me where I knew them from. If they susequently joined LinkedIn, then I'd pile on to connect with them. I rarely invite people. It's just to much hassle explaining it, cajoling them, and convincing them. If some one joins on their own, it is a done deal to do the old LinkedIn version of the Vulcan mind meld with them. So routinely dumping in large numbers of email address was a as needed task. Let them "stew" and once a week see who was "discovered".
Now LinkedIn doesn't let me put in "other contacts" so easily. Defeating an easy way to build a network. Arghh!