be in on the ground floor

To become involved in something at its inception. We're currently looking for investors for our exciting new project, so if you'd like to be in on the ground floor, be sure to let us know. After her wild success with her first product, many people wanted to be in on the ground floor for her next one. No one wanted to be in on the ground floor, but now that I've found success, they're all clamoring to join my company.

be, come, get, etc. in on the ground ˈfloor

(informal) become involved at the beginning of a plan, a company, an organization, etc. and possibly profit from this later: Reg came in on the ground floor and saw the value of his investment double in two years.