わせる(あわせる) To join together physical things (jigsaw puzzle pieces), or connect things (bands tuning up is called 音合わせ), or introduce two people.   $$$
わせる(ちからをあわせる) Use this for different social groups uniting for change.
併せる(あわせる) Much more specific and useless than the other あわせ (合わせ): 併せ is only used to mean “treat these two issues as one larger issue” or “handle both these matters as one item on the agenda.” You know what? Why don’t you forget about this one altogether.
(けっそく) Social unity:voting, marching, agitating for repeal of a law, etc. Unite and fight the Man! The Thai community is very close-knit in this town. Etc. Basically the noun form of 力を合わせる。