Abundance

Yes, they will get the benefit as well, if your Abundance affects their sites!

Mana is added to a player's mana pool during their Start Phase, or when a site enters the realm under their control. So, any sites already in play won't get the extra mana until the start of your next turn. But, if you play and then play a site into Abundance's area of effect, the newly played site will generate extra mana immediately. Also, if you start your turn with a site in Abundance, and then during your the site is no longer in Abundance (e.g. moving away, , etc.), you don't the mana that you gained during your Start Phase.