Teamtalk gives you a closed user group instant messaging system. You don't have to manage individual contact lists, everyone in the team sees everyone else. The administrator invites people to the group, and manages other settings for the account.
Email invitations are sent out to members of the team, by the administrator. After that team members join the group where they can set their name and status and use all the other features for collaboration.
Your teamtalk account is linked to your email address along with a password that you choose. This also decides which group you belong to. You can manage your settings online, with direct links from your client.