More typically a co-op will raise funds from members via shares. I'm a bit shaky on specifics, but those shares may be capable of being sold back (to the co-op as a whole). Voting righs often _don't_ accrue per share.
Debt financing, as mentioned, is another alternative.
The problem with that is there are tax implications to giving someone a chunk of a company (it's technically income), even if it is completely illiquid. Tax implications the employee probably can't afford much better than a buy-in.
coops normally have special status to get round some of these issues.
And coop members can have direct or indirect ownership a full on worker coop normally has direct ownership and organizations like John Lewis have indirect ie shares held in trust.
Debt financing, as mentioned, is another alternative.
Read with caution, I'm really sketchy on details.