When you hit the level 20-25 zone, you get a few quests that say they recommend 3 players. I think two would be enough for those. When i did them with my trio, they were very easy. There might be quests later on that are full group quests, not sure.Also, the Rifts are not forced grouping,, you have a choice to join the raid when you are near an area invasion or rift. But its smart to join, i believe you get better rewards for being in the raid, as well as more buffs and such.