Below the top spot where you can buy a Heroes III Alliance Member is a number of different options for spending Heroes III PlayMesh Points. Two slots below the normal Heroes III Alliance Member purchase spot is an option to spend twenty Heroes III PlayMesh Points for two Heroes III Alliance Members.
The process of spending the Heroes III PlayMesh Points in this situation works the exact same way as the previous purchase of Alliance Members, provided you have enough Heroes III PlayMesh Points. This is always going to be the best use of your Heroes III PlayMesh Points and you should try to direct most of them here. The best way to do this is to save up all of your Heroes III PlayMesh Points and then buy up Heroes III Alliance Members once they become unreasonably expensive.