Depends on the cooperative and the situation - there is no silver-bullet fix on how to fund cooperatives, but rather the solution lies in providing large variety of different options for cooperatives to choose from to best suit their situation.
Ideally of course cooperatives would never need investment or even a loan to start and operate, and the members would provide all the funding needed. But in the imperfect reality the choice people can face is between a conventional capitalist company or a cooperative with investor shares. This is good to remember as the discussion about investor shares in cooperatives is often on whether a cooperative with or without investor shares is more ideal.
There are situations where a loan makes sense and other situations where receiving an investment can be a better option.
If a group of 10 people have an idea for a venture that needs 100,000$ in seed-capital with a 1 in 10 chance to start making a surplus in 3 years, it’s unreasonable to ask the founder members to take the risk of most likely losing 10,000$ each and 3 years of work. You also cannot expect a credit union to risk their members money to a loan that is most likely not paid back. Turning down every single one of such ventures and letting them operate as capitalist firms is a choice the cooperative movement cannot afford to take if it wants to challenge the capitalist system.
I’m afraid the lack of sources of funding like investor shares have made cooperatives rare in some important areas of the economy where they are desperately needed. Even though we must do things fundamentally differently, there is a need for something that resembles venture capital in terms of funding cooperatives that require taking risks that are not reasonable to ask for the early members alone to take, but that can attract other people willing to take the risk in exchange for a possibility of a return in the future. Otherwise, certain type of ventures will be formed as capitalist firms instead of cooperatives.
If there are investors, ideally they have no voting rights, and if they do, ideally the voting power is shared equally, not to a proportion to their investment. Otherwise the first cooperative principle of one-member-one-vote is broken. It can therefore be argued that it if a cooperative has investors with voting power in proportion to their investment, it’s not a cooperative. On the other hand it could be argued that if the investors only hold 10% of the votes, it’s 90% cooperative and 10% capitalist. Therefore the case can also be made that it’s a cooperative with capitalist characteristics, not the other way around. If the choice is between a 100% capitalist firm and 10% capitalist firm, we should choose the latter. Another factor is the number of investors - having 100 investors having similar amount of voting power is closer to one member one vote than 1 investor that has more voting power than the 99 other investors combined.
Ideally the loan and the investment comes through a cooperative - so the loan comes from a credit union or a Coop Loan Fund and the investment through a cooperatively owned crowd-investment site solely dedicated for non-voting investor shares cooperatives like Coop Exchange. This way the returns are circulated in the cooperative ecosystem, instead of cooperatives paying part of their returns to capitalist firms.