General:
* Collaborate
* Cooperate
* Work together
* Team up
* Combine forces
* Join forces
* Unite
* Consolidate
More specific:
* Concur (agree and act together)
* Conspire (plan together secretly)
* Coalesce (come together and form a whole)
* Syndicate (form a group for business purposes)
* Consortium (a group of companies working together on a project)
* Alliance (a formal agreement between groups to cooperate)
The best word will depend on the specific context of your sentence.