Do clerics gain XP for building congregations (or other such activity?).
Only indirectly, via expending the Divine Power collected into magical research (pg 147)
That being said, I've been wondering about a cleric that would focus on building congregations at the expense of their personal domain/realm (or, something analagous to the difference between a bishopric and a bishop's personal domain, a'la the HRE) and allowing unspent Divine Power to collect as either Reserve XP (which I like because it's sort of an indirect "blessing" for the next character) or just directly as domain XP. Haven't done any math to see if that would be madness or not.