Good question! A membership to DG is free and provides you access to the basic features of the Plants Database, as well as access to the Garden Watchdog, the Garden Bookworm, your own personal garden journal, seed/plant trading tools, and an optional nightly newsletter. Non-subscribed members do not have the ability to read or post to our discussion forums, and they have limited access to the Plants Database (i.e., limited number of searches per day, and no access to the advanced search and browse features.
A subscription gives you unlimited access to the Plants Database, including the advanced search and browse features, plus access to the discussion forums, and the twice-weekly chat sessions. Some co-op opportunities are restricted to subscribers-only, at the discretion of the organizer. Some subscribers will offer their choicest plants in trade only with other subscribers (again, that's a personal decision.) Subscriptions are $15 per year, or $5 for a two-month trial.
Brugie's right - logout, and you'll see a link to the "registration page" in the right-hand column. Your friend will need to go to that page, type in their choice of username and a valid email address. The system will generate and send to them an initial password, which they'll be prompted to change when they sign in.