Do summoned creatures take a share of the XP of an encounter? Let’s say a PC in a party of 3 uses a spell like Conjure Animals. He uses it to summon all 8 creatures. If they contribute to the combat, is the XP split by 3 or is it split by 11?
Summoned creatures do not gain XP. As @Mindwin said in an answer to a different question:
They are part of a class feature of the character, and considered as "part" of that character. They are not NPC, they are creatures under the control of the PC with strict guidelines.
If a player summons additional help to aid the fight, they are using their own abilities to do so. Usually, in these cases that is what they are meant to do. A Druid or a Ranger don't have particularly high combat abilities, compared to other classes like the Fighter or the Barbarian, so they instead summon help, to assist.
They are using their skills to aid the fight, therefore they gain the experience for doing so, not the animals for making the kills.