You can use Kazarma's Web interface to search for Matrix and ActivityPub users. This quickly gives you the address to reach them from the other network.
📄️ ActivityPub user rooms
You can join Matrix rooms that show all public activities created by an ActivityPub actor (user, group, channel…).
📄️ Matrix user rooms
You can create a room where all messages that you create are forwarded as public Notes.
📄️ Direct messages
Direct messages, as they exist in Mastodon and Pleroma, are a way to exchange private messages between microblogging users, by using the "private" (or "direct") visibility of posts (the posts are sent only to the mentioned users).
📄️ Chat (Pleroma)
Kazarma is compatible with the chat system in Pleroma.
You can invite a Matrix user to a Mobilizon group. This will create a private room and invite the user. If they join the room, they will accept the group invitation.