入股 means 'to become a shareholder' or 'to invest by buying shares'. It is a separable verb (离合词): the two characters 入 and 股 can be split to insert 了, 过, or a measure word like 一股 (as in 入了一股 'bought one share'), or the object can appear between them (入某公司的股). You can also use it as an unsplit compound before the company name, e.g. 入股一家公司.