How do I import existing members to the database?

I've searched this topic, but I'm not sure I've really seen a straightforward answer on this.

I set up Membership for our existing organization. We've got about 150 members. We're going to be using the site as our membership management system from now on, so I need to get those 150 existing members into the database.

It's a pretty small number, so I don't mind putting them in one at a time if that's what I have to do. It seems like this would be a very basic and entirely reasonable thing to expect from a plugin called "Membership", but I'm not really seeing a good way to do it.