Add members
For chats that are attached to a group or community, members must be added directly to the group/community. You can find out how to do this here.
If you are in a multi-chat, you can add more members by clicking on the three dots at the top right of the chat.
Removing members
For chats that are attached to a group or community, the members must be removed directly from the group/community. You can find out how to do this here.
If you are in a multi-chat, you can click on the three dots in the top right-hand corner of the chat and then click on the three dots on the right-hand side of the desired member to remove it from the chat.