There's no formal rules, but I've found that its relatively easy to add some minions/lieutenants to the scene on the side of the heroes. If it makes sense in your story and your group members are using these contacts frequently, then maybe scale up the baddies to be a little harder (adding more of them, higher dice pools, stronger abilities, etc.). Many of the scenes in the starter kit are (H) minions and (H)/2 minions; you could easily do something similar and just do (H) + 1 or whatever.
Personally, I've only ran one game outside of the starter kit thus far, and I had an ally that I used as a balancing mechanic. Depending on how well the group was doing when entering the yellow and red zones, I added a heroic minion (happened to be a d12 cuz they werent doing so great) and it added both to the story and mechanics so I would reccomend doing something similar if needed.