Oil giant BP Amoco and a group of unnamed investors announced yesterday that they will invest as much as $100 million in GreenMountain.com, a company that sells renewable energy and natural gas as alternatives to power from dirtier sources. GreenMountain, which already has more than 100,000 residential customers in California, New Jersey, and Pennsylvania, plans to use the influx of money to expand into Ohio, Texas, and other states that are deregulating their energy markets. Under the deal, BP gets the right to market the GreenMountain brands of electricity to large business customers. GreenMountain, which plans to go public in about a year, also announced a deal yesterday under which its products will be promoted by the Internet search engine Lycos.