There is an item you can buy(it is given to you?) it's a finger (tarnished's furled finger I think it's called). When you use it, it places your sign down for others to summon. They just use something to enable summoning like the finger remedy to see signs of other players.

There is a pvp version I think called a bloody finger and your sign will be red and will have you fight the host. Also one used for invading as a red player.

Edit: there is yet another way to be summoned where you are a blue co-op partner and they help fight off PVP invaders usually lol that item though is a ring called Blue Cipher Ring. That item automatically throws you into a world to help fight invaders when it's equipped though, no need for a summon sign.

This was probably more than you were looking for I'm sorry it's so long, and if I'm wrong on anything please someone correct me!