Go to your options and type in the field for password: "SUNBRO" (without ") and set you summoning sign at the agreed location, that it is.
Because of the password, there no spliting of SL and NG+ between a SL 200+ Sunbro in NG+5 and a new player, for example.
I don´t have the intesion to split the community, but it is hard to find players when you have a high SL in the late game Around 200+ or when you are in a high NG+. This is mainly for people in the late game but could also be helpful for those in the early game who want help from people who know what they are doing.
I´m know that pvp also use passwords, so if you just want someone to help you, stay away from red, orange or purpel (sorry mundmarkers) sings.
Here is a link to a simlar "guide": https://www.reddit.com/r/darksouls3/comments/4g8ihv/darksouls_3_coop_password/

As a request from some user I createt a Steam groupe for the Sunbro Hotline. http://steamproxy.net/groups/sunbrohotline

Feel free to join it.
I recommend to start "discussion" for the areas, so it will have something like a filtersystem.