Whether or not you need to join (become a member of) a chat room depends on which privacy level the room has:
Open No membership is required (or allowed) to find, enter, post to, or follow an open chat room.
Closed Membership is required to enter, post to, or follow a closed chat room; no membership is required to find one.
Secret Membership is required to find, enter, post to, or follow a secret chat room.
Membership in a closed or secret room needs to be granted by a manager of the room. Information about secret chat rooms doesn’t come up in search unless you’re already a member.
Determine the privacy level of the room
In the Skype for Business (Lync) main window, use search to call up the listing for the room you want to participate in.
Point at the chat room listing and click the See contact card icon. You’ll see the privacy level and the manager’s name displayed on the contact card.
To learn more about Skype for Business (Lync) chat room privacy levels and membership, see Understanding Lync chat room membership.