More players means SR3MP

What SR3MP is

The Saints Row Mods thread and the SR3-MP GitHub describe one project, so I count them as 1 source, not two independent mods. You need a legitimate Remastered install. Modified game files are called out as incompatible. It does not add a third player to the story missions. Official co-op remains two players and is explained on the co-op page.

“Coop mod” in search logs may be this sandbox or a wish for campaign co-op. I am answering with the project that exists. If a different co-op dll appears later, this page should change. Today the mapping is SR3MP.

Abandoned status

An abandoned multiplayer server is not something I will tell you to port-forward on faith. Read the thread’s last dates before you open a port. I am not running a server list.

Campaign co-op versus a sandbox server

Decide which experience you are looking for before installing anything. The retail campaign has shared story objectives for two players. SR3MP is a separate multiplayer project with server behavior and a sandbox focus. A server slot count does not imply that campaign scripts, activity rewards, mission progression or save compatibility have been extended to that number of players. Use the project’s own server instructions and supported build, and keep its files separate from a regular campaign installation. For an ordinary story session with one friend, the retail co-op guide is the relevant starting point.