Looking at the code, I believe this is the minimum number of people who must initiate an invite for it to take effect. E.g. with the settings shown in the example screenshot, (the defaults) Bob and Susan must both invite Fred and the combined trust level of Bob and Susan must be 4 or higher.
If Susan is trust level 4, she would meet the requirement for the trust level on her own but the minimum number of inviters would not be met, so Bob would still need to invite Fred before it takes effect.