Assign multiples users to read and edit a item

heres an example using pnp-js-core:

// get the role definitions
await pnp.sp.web.roleDefinitions.get().then((roleDefs) => {
this.addMessage("got roledefinitions");
roleDefinitions = roleDefs;
return;
}).catch(error => {
debugger;
this.addMessage("<h1>error fetching roledefs</h1>");
this.addMessage(error.data.responseBody["odata.error"].message.value);
console.error(error);
return;
});
// get the site Groups
await pnp.sp.web.siteGroups.get().then((sg) => {
this.addMessage("got Site Groups");
siteGroups = sg;
return;
}).catch(error => {
debugger;
this.addMessage("<h1>error getting site groups</h1>");
this.addMessage(error.data.responseBody["odata.error"].message.value);
console.error(error);
return;
});

// Setup security on the library. First, break role inheritance
await list.breakRoleInheritance(false).then((e) => {
this.addMessage("broke role inheritance on " + library["Title"]);
}).catch(error => {
debugger;
this.addMessage("<h1>error breaking role inheritance on library " + library["Title"] + "</h1>");
this.addMessage(error.data.responseBody["odata.error"].message.value);
console.error(error);
return;
});
// second , add the library-specific group
let group = find(siteGroups, (sg => { return sg["Title"] === library["EFRsecurityGroup"]; }));
let principlaID = group["Id"];
let roledef = find(roleDefinitions, (rd => { return rd["Name"] === "Content Authors without delete or modify"; }));
let roleDefId = roledef["Id"];
await list.roleAssignments.add(principlaID, roleDefId).then(() => {
this.addMessage("granted " + library["EFRsecurityGroup"] + " read access to " + library["Title"]);
}).catch(error => {
debugger;
this.addMessage("<h1>error adding role asisigment to library " + library["Title"] + "</h1>");
this.addMessage(error.data.responseBody["odata.error"].message.value);
console.error(error);
return;
});

IIn this case I am assigning a group to a list, but the same nethod would be used to assign a user to a file/listitem. You would just need to get siteUsers instead of siteGroups.