* Different ways to group letters? Like alphabetically, by frequency, by sound, or by meaning?
* Different types of letters used in different languages? Like the Latin alphabet, Cyrillic alphabet, or Greek alphabet?
* Letters that are grouped together in a specific way to represent a sound? Like digraphs (sh, th) or trigraphs (tch, sch)?
* Letters that form a specific group in a word? Like the "ph" in "phone" or the "ch" in "chair"?
Once you tell me what you mean by "group letters," I can provide you with a more helpful answer!