To add members:
Open the Conversation.
Tap Members.
In the bottom bar, tap .
In the Add Members dialog, select the people or groups you want to add.
By default, you see Recommendations.
To view a list of your contacts, tap Contacts.
To view a list of your groups, tap More, then select the list you want to see.
Alternatively, search for a person or group to add.
To view the list of people and groups you've selected, tap > next to <Number> selected.
Note: People outside your company have a special avatar () and their names are in purple text. If you add an outside user to a Conversation, a banner stating "Visible to outside users." appears at the top of the Conversation and any associated message dialogs.
Tap Add.
To remove members: