Probably the most common way to invoke a Lambda is to use API Gateway, which maps a REST call to the lambda code. You can add multiple triggers to your Lambda during at any time, including when creating a new lambda.
If you are familiar with API Gateway, you can associate any method with a deployed lambda. Simply create a new method with Integration type set to Lambda function and point to your lambda. You can map both REST inputs to lambda inputs and outputs to REST outputs.
TODO: Decide whether to describe API Gateway here or reference another part of documentation.