How to encourage students to answer their peers' questions
My classes have a similar online forum (Piazza).

The best way I’ve found to encourage student participation is to have an explicit policy, published in the course syllabus, that the TAs and I will not answer any question about course content in less than four hours.  Not “may not”, but “will not”. (Questions about course administration or potential errors in lectures/notes/homework/labs/exams/solutions are exempt; we answer those as fast as we can.  We also try to upvote good student answers on course content quickly.)

I also offer extra credit for especially helpful questions or answers.  (Notice I said “helpful”, not “numerous”; it’s important to reward quality, not quantity.)  But I have the sense that this helps considerably less than just getting out of the way.

It’s also important to encourage a supportive and collaborative environment elsewhere in the class.  So among other things, I encourage students to work together: in small teams (up to 3) to write homework, in larger groups in discussion sections, and with whatever groups they want to figure out the homework, study for exams, and so on.  (Homework teams that work together are still required to write solutions independently, and they are required to cite each other.)  So students are already used to helping each other offline.