get in on the ground floor
get in on the ground floor {v. phr.}
To be one of the first members or employees to participate in the growth of a firm, educational institution, etc.
Elliott got in on the ground floor and made a fortune at the company.
Mr. Smith who joined the new college as an instructor, got in on the ground floor, and wound up as its president after twenty years.
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Source: A Dictionary of American Idioms