Formula to create comma separated string

If the data is in single row means there are 3 or more columns then you can do this in the Report header as.
Stringvar abd := "";
adb:= field_1 + ", " + field_2 + ", " ......

If the record is in single row with multiple colums then you have to create a subreport. And in its detail section do as

shared stringvar abc;
if length(abc) > 0 then
abc := abc + ", " + field
Else
abc := field;

as the whole data is stored in shared variable.

Now go to the report where you want to show this data and create a formul like show and write as
shared stringvar abc.

Keep this in mind to place the subreport before the section or subreport where you are showing this data. Because first it will be evaluated then it canhave the data to show.

join({Your_field},",")

Create a sub report and put it before the section where you want to show the names.

In the subreport lets call it concat_names

create a shared stringvar names := "";

in the detail section of the concat_names subreport put a formula and do as in it

shared stringvar names ;
if Length(names) > 0 Then
names := names + ", " + {s_FirstName};
else
names := {s_FirstName};

now go to the main report where you want to show the data create a formula named show_names
and simply write
shared stringvar names;

Keep in mind to place the concat_names subreport before the section where you want to show the names.

check the attached file