If you have a parameter array as the following:
 

Parameters!ParameterName.Label(0) = "Hello"

Parameters!ParameterName.Label(1) = "Parameters"

Parameters!ParameterName.Label(2) = "World"
 

With the Join function In the F(x) Expression
 

=Join(Parameters!ParameterName.Label, " ")
 

The result would be a string like the following one:
 

"Hello Parameters World".
 

You also can use any other separator:
 

=Join(Parameters!ParameterName.Label, ", ")
 

And the result would be:
 

"Hello, Parameters, World".