The first thing to understand is さえ. さえ is a particle which essentially creates a focus on the thing which proceeds it, and is usually translated as “even”. It can attach to a noun or to a verb, depending on where the emphasis is placed, and is used in a variety of constructions (I don’t go over them all here; see e.g. Maggie Sensei’s post on this http://maggiesensei.com/2015/03/28/how-to-use-さえsae/ for something more comprehensive!). For example:
He is afraid even of his own shadow.
Somewhat logically, V+さえすれば is a set expression which means “if only”, “all you have to do”, “as long as” or similar. The ば is indeed the conditional form here. Context will help decide which sounds most natural in English. For example:
All you have to do is wash the dishes. (lit. As long as you just wash the dishes, it will be good)
Thus, 二人以上の人が顔を合わせさえすれば looks like it should mean something like “as long as at least two people meet(/face each other)”. Again, the most natural translation may be different depending on context.
Regarding ば vs たら (and other types of conditional!), those are well explained here Differences among -たら、なら、-んだったら、-えば, etc.
Hope that helps.