General:
* Unite: To join together as one; to combine into a single group.
* Combine: To join or merge together.
* Consolidate: To make something stronger or more effective by combining parts.
* Integrate: To combine or blend into a whole.
* Amalgamate: To combine or unite, especially to form a single organization.
Specific:
* Coalesce: To come together to form a single mass.
* Assemble: To bring together in one place.
* Congregate: To gather together in a group.
* Aggregate: To collect or gather together.
* Synthesize: To combine different ideas or elements into a new whole.
The best word to use will depend on the specific situation.