It seems there is some sort of a bug with the invitation thingy. If you invite, via e-mail, a "friend" to join BitBucket and he/she actually joins you get 1 free user with your plan (at most 3 extra users). If that friend decided to drop the BitBucket account, for whatever reason, you can send the invite again and if that same persone, with the same email, joins BitBucket again you get another user for practically nothing.
Basically one can create an temporary e-mail and send an invitation to that e-mail. The so called "friend" (the one who sent the mail actually) joins BitBucket and bingo! One free user added to the plan. Repeat the step 2 more times and get a total of 3 extra without actually inviting anyone to BitBucket. The temporary e-mail can be forgotten or deleted (if the E-Mail service provider allows it).
The cheat actually makes sense when there are already 8 people registered and wish to form a team and work togeder at some "secret project" and require private repos and they are on BitBucket before the invites thingy. There's no one else to invite or no one will bother to answer such invites for random reasons (some have accs on github, some don't want to work with git/mercurial, some work on codeproject etc.). In this case you the cheat is useful because you can get all 8 members without having anyone actually join.
PS: I know it doesn't seem important, some may go like "meh, it's only 3 users, get over it". I just wanted to post it (I didn't check if the bug was already reported, sorry if it is).
PS2: I got all 3 users for a friend and I covered the 3rd for me :)