A team blog will only switch to Blogger in beta when the original owner moves his or her account over.
Team members who have not yet switched to Blogger in beta will still see the blog on their dashboards, but it will be grayed out and inaccessible. Once those members move their own accounts to the beta, they will be able to access the blog as before. If they can't yet switch, or don't wish to, they can ask the owner of the blog for a new invitation to join the blog on the beta version. They can then accept that invitation by logging in to or creating a Google Account to post to that blog. (Later on, when they do switch their Blogger account to the new version, they'll have an option to merge it with this account.)
If a team member of a blog switches their account to beta before the blog owner does, then the team blog is not converted. It will appear on the team member's dashboard in beta, but clicking on it will lead back to the old version of Blogger. Thus, the blog will still be usable, but without the new features of Blogger in beta (e.g. Labels, Layouts, etc.).