You are confusing between operators and cmdlets that are present in Powershell with methods and properties of (.NET) objects. gm
or Get-Member
will only show the latter.
You can learn about -join
by doing get-help about_join
or look here - http://technet.microsoft.com/en-us/library/hh847757.aspx
Take a look at about_Operator
to learn about other available operators - http://technet.microsoft.com/en-us/library/hh847732.aspx