PLEASE USE THE 1.2.1 RELEASE, THIS ONE BROKE ASYNC SOCIAL EVENTS.

Changelog:

Bundle sample projects in the repository. (and also attached to this release)
Switch to GmxGen for sample projects. (bless YellowAfterlife)
Added libmulti_mouse_get_x and libmulti_mouse_get_y to get mouse coordinates.
Added libmulti_set_window_style and libmulti_get_window_style to change window styles at runtime.
Added libmulti_set_min_width, libmulti_set_min_height, libmulti_set_max_width, libmulti_set_max_height, libmulti_set_min_size. libmulti_set_max_size, libmulti_get_min_width, libmulti_get_min_height, libmulti_get_max_width, libmulti_get_max_height functions to manage the minimal and maximum allowed sizes of the window at runtime.
Some hacky GM8.1 support (not in the compiled builds yet, needs a rewrite tbh). You should seriously consider switching to Studio anyway.
Functions to set the window icon are planned, but are not implemented yet. Once I figure it out I'll make a 1.3 release.

GM:S 1.4 users: Import the libmulti.gmz project and look at the sample object.
GMS 2.2.5 users: Import the libmulti.gms2.yyz project and look at the sample object.
GMS 2.3.1+ users: Import the libmulti.gms23.yyz project and look at the sample object.

This implies you've already imported and tinkered with the sample project. (oh and also read the docs a bit)

Load your game project.
Right click at Extensions then click Add Existing
Then in the opened up file dialog, navigate to the extensions folder in the sample libmulti project
And then choose the libmulti.extension.gmx file if you're using GM:S 1.4 or the libmulti.yy file if you use GMS 2+.
A new extension called libmulti should be added, then you're good to go!