blockslack is a decentralized group communication tool. You sign in using a Blockstack ID; all of your communication data is owned by you and stored in the hub associated with your ID.
You can create groups for the various different contexts that you communicate in (maybe have one group for a company, one group for friends and family and another for a side project you are working on). Within groups you create chat channels; you can invite any Blockstack user to any channel.
Anyone in a channel can add new members, or remove existing ones. There is no concept of ownership at the group or channel level; every member owns the messages that they post, and publishes them in a way so that they can only be read by people who were members of the channel at the time of publishing.