General Antonyms:
* Company
* Togetherness
* Community
* Socialization
* Interaction
* Association
Antonyms emphasizing human connection:
* Friendship
* Love
* Camaraderie
* Intimacy
* Partnership
Antonyms emphasizing activity:
* Bustle
* Hullabaloo
* Activity
* Congregation
* Gathering
The best antonym will depend on the specific meaning of "solitude" in the context of your sentence.