At present I have two games where the core idea of the "story" is to be about a unit of soldiers and the bonds of brotherhood/sisterhood that form between the members. Both games, due to this focus, have a mechanic in place to represent how well the group works together and to represent the growth of that bond of soldierly familiarity that is such a staple of the military drama genre. With this in mind, and with the idea of helping anyone considering making a similar game of their own, let's talk about what should go into these systems.
Cohesion Grows With Time
This is fairly basic, but when you just throw a group of people who don't know each other together, they probably won't work all that well together. Oh sure, maybe they can pull off some teamwork. Maybe the sniper and medic get a long and do some fun stuff together. By and large though, the group doesn't know each other, and so it isn't going to work as well as say a veteran team that's done 50+ deep cover missions with each other. This is one of the first things that your mechanic should address, the bond grows with time. Late game it may even be somewhat broken, but that's ok as long as you can represent this core feature.
It Is All About Trust
A group's dynamics generally revolve around trust. If you trust your group, you can work well with them. If you don't; you won't. Acts of betrayal, acts of cowardice, and things that work against the group for personal gain are things you may want to consider hindering the group's cohesion. After all, the team isn't going to work as well together if suddenly the guy you thought you knew is backstabbing you in the field or sleeping with your wife, right?
Focus On The Group
The idea of this mechanic is the group, and as such the group should be the most important part of whatever this mechanic is. No one person should be more important than others to this mechanic. Everyone should contribute, and the contributions should match how their character has contributed to the group. Doing this lets you naturally have mechanics that reflect members leaving the group and joining. Essentially, as the group changes so should how this mechanic effects them - or at least the degree to how it affects them.
Other Things To Consider
Some other things to consider for a mechanic like this could be the effects of a strong leader, who can make a group work together even if they seem an unlikely combination; the effects a morale-centric character can have, such as the medic who makes sure everyone is as happy/comfortable as can be; and anyone else who focuses on the group more than themselves. Heroic actions should be rewarded, and often these mechanics are the place to do just that.
What Would You Want Considered?
What would you want in one of these mechanics? Have you seen it done especially well in any game? So far I think my favorite representation is what John Wick's Aegis Project has, but I'm always re-evaluating those likes with new information. Sound off in the comments.