(さんせい) Agree (more informal). Can also mean to support, as in “Many people support this new law.” “Most of the committee members support the chairman’s fuckin’ proposal.” (in the case of “support” it is usually used about a great number of people, not just one individual)
(どうい) Agree (more formal). Usually used about serious matters such as contracts and organ transplants. When you click on a EULA, you 同意. You wouldn’t 同意 about  which restaurant to go to with friends, you’d 賛成.