"민수와 위라가 되어 친구와 이야기해 보세요."
=> (lit) Now become 민수 and 위라, and try talking with your friend.
=> Try talking to your buddy/classmate (play-acting) as 민수 and 위라.
(The text shows the characters of 민수 and 위라, and now it's asking the readers/students to talk to each other as if they are those characters.)
* X-이/가 되다 = to become X.
* 되어 = short for 되어서. -서 indicates a 1) reason/grounds for an event, or 2) doing something as a basis for the next (more important) action (the case here). E.g. 나머지 숙제는 집에 가서 할게요 would be "I'll do the rest of the homework at home = (lit) I'll go home and do it".
* -아/어 보다 (이야기해 보세요) is a common phrase for doing something to see what happens or as a lighthearted trial. E.g. 이거 먹어 봐. 맛있어 = Have a taste of this. It's good.