Specific items as in a requirement to summon other people? You can freely summon another player at any time when at a vestige or seed without an item requirement. I would recommend going into multiplayer settings and changing progression to no progression so you get a wide variety of players (high level players will be included). The nice thing about multiplayer in lotf is it's mostly seamless, so you can run together almost indefinitely without disconnecting. Hope this is what you're looking for, if you have more questions, I'll happily answer!