To add permissions to groups you first need to be a Superadmin,

You can add groups using the /addgroup command. So for example:

/group add awesome

This will add a new group called "awesome"

To add users to the group:

/user group Multiplay awesome

This will add the Multiplay User to the group awesome.

Spawning bosses!

To add the permission to spawn bosses to this group:

/group addperm awesome tshock.npc.spawnboss

This add the permission tshock.npc.spawnboss to the group awesome, then the user Multiplay is able to spawn bosses with the commands:

/eater /eye /king /skeletron /wof(/wallofflesh) /twins /destroyer /skeletronp(/prime) /hardcore

Full list of permissions: