Here are some options you could use, depending on the context:
General terms:
* Group
* Team
* Mission (This emphasizes their purpose)
* Band (This implies a united and dedicated group)
More specific terms (if applicable):
* Convent (for a group of nuns)
* Brotherhood (for a group of monks or friars)
* Order (for a religious organization with a specific mission)
Ultimately, the best collective noun depends on the specific situation and the tone you want to convey.