WordNet sense 1 (be or become joined or united or linked):

join, link, unite

WordNet sense 2 (establish communication with someone):

get in touch, touch base

WordNet sense 3 (plug into an outlet):

plug in

WordNet sense 4 (connect, fasten, or put together two or more pieces):

tie

WordNet sense 5 (make a logical or causal connection):

associate, relate, colligate, associate, tie in