Hi, another newbie!
Easy to add the subscription but then I decided I wanted some kind of digest so I am not inundated by messages throughout the day.
I am not sure about the wording of the option - Group messages together as one email instead of individually.
I am assuming you mean "daily digest". All the day's messages come together. 
Perhaps you don't want to use that term, but perhaps something such as:

Get daily messages as one email instead of receiving individual emails. Maximum of one email per day
or
Get the day's messages in one email instead of individually. 
or 
Daily digest: Group messages together as one email instead of individually.

Otherwise, it isn't clear how often you get them, if there is traffic every day. Grouping also just sounds like threading which some people hate!