Unite, combine, conjoin, cooperate, concur are comparable especially when they are applied to people or groups or categories of people or to human institutions and when they mean to join forces so as to act together or to form a larger unit.
Unite may suggest either of these ends, but it more commonly suggests the formation of a new or larger unit (as by merging).
Combine (see also JOIN ) is often used interchangeably with unite , but it may be preferred when a somewhat looser or more temporary association is to be suggested.
Combine also is the one of these words that is at all likely to carry derogatory connotations (compare combine under COMBINATION ).
Conjoin (see also JOIN ) stresses firmness of the combination.
Cooperate implies a combining for the sake of action or mutual support or assistance.
Concur in this relation retains the notion of agreement from another of its senses (which see under AGREE 2 ) and specifically implies a joining in agreement.