Farcry 4 - How do I use hand signals (Salute, etc)
Solution 1:
You are looking for the Coop menu button. It is bound to 'V' by default. The binding can be changed by going to 'Keyboard and Mouse' (or Controller I suppose), then 'Customize controls' in the options.
Once you are in a co-op game it is used to bring up the coop action menu, as seen here.