Definitely Deku. His naivete and eagerness to learn would make for some really cute and funny group chat interactions. Bakugo would also be great. His hot - headedness and tendency to yell (even in a chat) would add a lot of drama. And All Might, he could be like the wise old mentor who drops in to give inspiring messages from time to time.
I think Todoroki would be very interesting. His complex backstory and his somewhat cold exterior would create an interesting contrast in the group chat. He might say something really deep or mysterious. Then there's Momo. She's smart and responsible, so she could be the one who tries to keep the chat on track when it gets wild. And of course, Mineta. His perverted nature would surely stir up some chaos and comedy in the group chat.