Yes. Initially I she tried to signup via bitbucket but then wasn't able to see any of the projects and the invite screen still showed her as pending. So I deleted the pending invite and invited her again. Now she only sees the login screen but doesn't have a password.
If you're adding someone that already has a login, then they should log in, after which they'll be able to switch between their own account and other accounts to which they belong on the upper left of UI.