diff --git a/src/bot.rs b/src/bot.rs
index a687551..0aaf6f4 100644
--- a/src/bot.rs
+++ b/src/bot.rs
@@ -11,9 +11,9 @@ use matrix_sdk::Error as MatrixError;
 use matrix_sdk::{
     self,
     events::{
-        room::member::MemberEventContent,
+        room::member::{MemberEventContent, MembershipState},
         room::message::{MessageEventContent, NoticeMessageEventContent, TextMessageEventContent},
-        AnyMessageEventContent, StrippedStateEvent, SyncMessageEvent,
+        AnyMessageEventContent, StrippedStateEvent, SyncMessageEvent, SyncStateEvent,
     },
     Client, ClientConfig, EventEmitter, JsonStore, Room, SyncRoom, SyncSettings,
 };
@@ -229,6 +229,37 @@ async fn should_process<'a>(
 /// Originally adapted from the matrix-rust-sdk examples.
 #[async_trait]
 impl EventEmitter for DiceBot {
+    async fn on_room_member(
+        &self,
+        room: SyncRoom,
+        room_member: &SyncStateEvent<MemberEventContent>,
+    ) {
+        //When joining a channel, we get join events from other users.
+        //content is MemberContent, and it has a membership type.
+
+        //Ignore if state_key is our username, because we only care about other users.
+        let event_affects_us = if let Some(our_user_id) = self.client.user_id().await {
+            room_member.state_key == our_user_id
+        } else {
+            false
+        };
+
+        let should_add = match room_member.content.membership {
+            MembershipState::Join => true,
+            MembershipState::Leave | MembershipState::Ban => false,
+            _ => return,
+        };
+
+        //if event affects us and is leave/ban, delete all our info.
+        //if event does not affect us, delete info only for that user.
+
+        //TODO replace with call to new db.rooms thing.
+        println!(
+            "member {} recorded with action {:?} to/from db.",
+            room_member.state_key, should_add
+        );
+    }
+
     async fn on_stripped_state_member(
         &self,
         room: SyncRoom,
diff --git a/src/db.rs b/src/db.rs
index 62a94e6..b667bd8 100644
--- a/src/db.rs
+++ b/src/db.rs
@@ -1,5 +1,6 @@
 use crate::db::errors::{DataError, MigrationError};
 use crate::db::migrations::{get_migration_version, Migrations};
+use crate::db::rooms::Rooms;
 use crate::db::variables::Variables;
 use log::info;
 use sled::{Config, Db};
@@ -8,6 +9,7 @@ use std::path::Path;
 pub mod data_migrations;
 pub mod errors;
 pub mod migrations;
+pub mod rooms;
 pub mod schema;
 pub mod variables;
 
@@ -16,6 +18,7 @@ pub struct Database {
     db: Db,
     pub(crate) variables: Variables,
     pub(crate) migrations: Migrations,
+    pub(crate) rooms: Rooms,
 }
 
 impl Database {
@@ -26,6 +29,7 @@ impl Database {
             db: db.clone(),
             variables: Variables::new(&db)?,
             migrations: Migrations(migrations),
+            rooms: Rooms::new(&db)?,
         })
     }
 
diff --git a/src/db/rooms.rs b/src/db/rooms.rs
new file mode 100644
index 0000000..3d507fb
--- /dev/null
+++ b/src/db/rooms.rs
@@ -0,0 +1,82 @@
+use crate::db::errors::DataError;
+use crate::db::schema::convert_i32;
+use byteorder::LittleEndian;
+use sled::transaction::{abort, TransactionalTree};
+use sled::Transactional;
+use sled::Tree;
+use std::collections::HashMap;
+use std::collections::HashSet;
+use std::convert::From;
+use std::str;
+use zerocopy::byteorder::I32;
+use zerocopy::AsBytes;
+
+#[derive(Clone)]
+pub struct Rooms {
+    /// Room ID -> RoomInfo struct (single entries)
+    pub(in crate::db) roomid_roominfo: Tree,
+
+    /// Room ID -> list of usernames in room.
+    pub(in crate::db) roomid_usernames: Tree,
+
+    /// Username -> list of room IDs user is in.
+    pub(in crate::db) username_roomids: Tree,
+}
+
+// /// Request soemthing by a username and room ID.
+// pub struct UserAndRoom<'a>(pub &'a str, pub &'a str);
+
+// fn to_vec(value: &UserAndRoom<'_>) -> Vec<u8> {
+//     let mut bytes = vec![];
+//     bytes.extend_from_slice(value.0.as_bytes());
+//     bytes.push(0xfe);
+//     bytes.extend_from_slice(value.1.as_bytes());
+//     bytes
+// }
+
+// impl From<UserAndRoom<'_>> for Vec<u8> {
+//     fn from(value: UserAndRoom) -> Vec<u8> {
+//         to_vec(&value)
+//     }
+// }
+
+// impl From<&UserAndRoom<'_>> for Vec<u8> {
+//     fn from(value: &UserAndRoom) -> Vec<u8> {
+//         to_vec(value)
+//     }
+// }
+
+impl Rooms {
+    pub(in crate::db) fn new(db: &sled::Db) -> Result<Rooms, sled::Error> {
+        Ok(Rooms {
+            roomid_roominfo: db.open_tree("roomid_roominfo")?,
+            roomid_usernames: db.open_tree("roomid_usernames")?,
+            username_roomids: db.open_tree("username_roomids")?,
+        })
+    }
+
+    pub fn add_user_to_room(&self, username: &str, room_id: &str) -> Result<(), DataError> {
+        //in txn:
+        //get or create list of users in room
+        //get or create list of rooms user is in
+        //deserialize/create set and add username to set for roomid
+        //deserialize/create set and add roomid to set for username
+        //store both again
+        let user_to_rooms: HashSet<String> = self
+            .username_roomids
+            .get(username.as_bytes())?
+            .map(|bytes| bincode::deserialize::<HashSet<String>>(&bytes))
+            .unwrap_or(Ok(HashSet::new()))?;
+
+        let room_to_users: HashSet<String> = self
+            .roomid_usernames
+            .get(room_id.as_bytes())?
+            .map(|bytes| bincode::deserialize::<HashSet<String>>(&bytes))
+            .unwrap_or(Ok(HashSet::new()))?;
+        Ok(())
+    }
+
+    pub fn remove_user_from_room(&self, username: &str, room_id: &str) -> Result<(), DataError> {
+        Ok(())
+    }
+}