to add; (math.) plus; to increase; to augment; (used before a disyllabic verb, often after an adverb like 不, 大, 稍 etc., to indicate that the action applies to sth previously mentioned, as in 稍加改良 "make some minor improvements to (it)")

to participate; to take part; to join

to add oil; to top up with gas; to refuel; to accelerate; to step on the gas; (fig.) to make an extra effort; to cheer sb on

to raise; to increase

to reinforce; to strengthen; to enhance

to accelerate; to speed up; to hasten

to become a member; to join; to mix into; to participate in; to add in

to work overtime