This idea is taken from Talk Less Teaching and I have found it to be extremely useful.
To encourage all members of a group to participate, use wool or string during the discussion. Every time a person speaks, they take the ball of string/wool. This tracks who is participating easily from a distance for the benefit of the teacher, who can view participants in the different groups and ensure every child is taking an active role in the group. It also has the benefit of making all pupils aware that the teacher can track their participation easily so they are more likely to feel under pressure to take an active role rather than sit back and let the other members of he group do all the work.
You could reward pupils (e.g. with raffle tickets that are used to enter them in a prize draw. Alternatively, with house points or equivalent) by giving each pupil a reward equivalent to their contribution, e.g. 4 raffle tickets for 4 contributions.