Choosing SHARES means that each participant in your co-op will get an equal "share" of however many affiliates your co-op generates. For example, if you have 10 participants and your co-op generates 500 affiliates, each participant will receive 50 affiliates. This is the purest form of a co-op; each participant takes an equal risk and shares in the results of the co-op, good or bad. Unless you are a very experienced marketer, we recommend SHARES.
Choosing SIGN-UPS means that you are guaranteeing each of your participants a specific number of affiliates—no more, no less. Be careful about selecting this option! Sometimes the best-planned advertising campaigns fail miserably. If you've guaranteed a certain number of affiliates, and your advertising does not produce them, YOU will be responsible for refunding your participants, making up the difference, etc.
Note that any affiliates your co-op would generate over and above the quantity earmarked for your participants will automatically flow into network with you as the Personal Sponsor. Example: You've got 10 participants and you've specified 20 affiliates for each. You co-op actually generates 300 affiliates. Hence, after your 10 participants received their 20 affiliates each (200 total), the remaining 100 affiliates would automatically flow to YOU.