Concatenating multiple values entered in a repeating table for use or display elsewhere in the form can be done with the following procedure.
First create a hidden field inside the table - e.g., {{tally}} - that uses a simple IF statement like this in the Dynamic Value property to determine whether a user input field within the table - e.g., {{userInput}} - contains data:
IF({{userInput}}!='',1,0)
Then in the field outside the table, use the JOIN function, adding a qualification with COUNT to only JOIN if there is more than 1 row in the table:
IF(COUNT({{tally}})>1,JOIN(', ',{{userInput}}),{{userInput}})
This will concatenate all instances of the input for {{userInput}} as long as at there are more than one row in the table, separating them by a comma and space after each one. If the value of {{tally}} is only 1 - i.e., if there was only one row added to the table - then only the one entry from the table will be shown, without any comma and space afterward.
Note that the comma and space used in this example can be substituted for another conjoining element like a hard return - i.e., /n - or otherwise.
For displaying repeating values from a repeating table on an output, please see the following article: Repeating Data