When talking/discussing in Japanese, there are times when we can't get involved in the conversation. So how can I participate?
First of all, when someone is speaking, nod (shake your head vertically). Some people nod their heads when talking one-on-one, but when discussing with many people, they sit still and do nothing. This way, the person speaking will feel insecure. Nodding is not only used when agreeing with the other person's opinion, but even when disagreeing, it is still a signal "I am listening to you", so just a slight nod is considered participating in the discussion. By doing so, the speaker can continue speaking with peace of mind. When nodding, you can reply with "hâi" or "sō desu ne".
Furthermore, in places where we truly sympathize (note 1), saying “that's true” is very important. Doing so will make it easier to insert yourself into the conversation.
(according to the article “Nodding to show sympathy”, Nihon Keizai Shimbun morning issue September 24, 2011)
(note 1) 共感する: feel or understand the other person's moods and opinions as your own.