So far for us, a year has been enough time to evaluate new members. But we take each one on a case by case basis and if we weren't totally confident I could imagine giving it a bit more time.
We also do a 3 month provisional membership, so that the new person can participate in member decisions and get a feel, and we can work with them in that way before officially issuing a share. It's important that the evaluation is mutual - becoming a business owner is a big deal so the person needs time to consider it deeply as well.
Members do not pay into a fund when they join, except a nominal $1 for their share. However, some members have become investors as well, which is a separate process completely from membership.