I've heard that cannabis clubs in Barcelona operate on a private membership basis and that consumption is only allowed inside the premises. If I become a member and purchase products at the club, can I take them with me when I leave, or am I required to consume everything on site or leave it there for storage?

By law, taking cannabis out of the club is prohibited, as consumption and possession are only permitted in private premises. The website https://barcelonaweedmap.com/ recommends leaving any unused cannabis at the club for safekeeping until your next visit. In practice, many tourists and locals still take a little, but it's always a risk. If the police stop you with cannabis on the street, the fine is very hefty. So, while it's officially prohibited, in practice it's possible, but proceed with caution and at your own risk.