There is a penalty with a henchman and other associates (the name for NPCs in the player's party) such as summons and familiars/animal companions are the same as henchmen in the default system. So, a PC playing along with a lone familiar will have the same 20% penalty that playing with a lone henchman would.
But it's slightly more nuanced than this. The XP per creature killed is lower with extra associates, with the percent penalties noted earlier. But, most encounters (those that are set up as encounters in the Toolset) will spawn more creatures when triggered by a bigger party, which can partly make up for the XP penalty per party member. For the most part, there is no increase in the number or difficulty of bosses (although that is possible with a boss fight set up through an encounter), so a player will almost always see lower overall XP when fighting with associates than without. But, because of the extra opponents to kill, the overall loss with extra associates is not always quite as bad as 20%, 36%, 49%, etc.