The word "unite" itself is a verb that already means "to join together" or "to combine".
If you're looking for words related to uniting, there are many prefixes and suffixes that can be used to create related terms:
* Prefixes:
* Re- (reinforce, reunite)
* Dis- (disunite)
* Uni- (unify, universal)
* Suffixes:
* -ion (union, unification)
* -er (uniter)
* -ed (united)
* -ing (uniting)
It depends on the specific meaning you're trying to convey!