diff --git a/cmd/welcome/main.go b/cmd/welcome/main.go
index 3a0ef22..5818ada 100644
--- a/cmd/welcome/main.go
+++ b/cmd/welcome/main.go
@@ -135,6 +135,10 @@ if you end up very stuck, you can email root@tilde.town .`, data.Username))
 
 	fmt.Println(s)
 
+	if err = invite.MarkUsed(inviteDB); err != nil {
+		return fmt.Errorf("could not mark invite as used: %w", err)
+	}
+
 	// TODO burn invite
 
 	return nil
diff --git a/invites/invites.go b/invites/invites.go
index f665ab7..2cd9f6d 100644
--- a/invites/invites.go
+++ b/invites/invites.go
@@ -3,6 +3,7 @@ package invites
 import (
 	"database/sql"
 	"encoding/base64"
+	"errors"
 	"math/rand"
 	"strings"
 	"time"
@@ -121,8 +122,25 @@ func Get(db *sql.DB, code string) (*Invite, error) {
 	return inv, nil
 }
 
-func IsCodeValid(db *sql.DB, code string) (bool, error) {
-	return false, nil
-}
+func (i *Invite) MarkUsed(db *sql.DB) (err error) {
+	var stmt *sql.Stmt
+	var result sql.Result
+	var rowsAffected int64
+	if stmt, err = db.Prepare(`UPDATE invites SET used = 1 WHERE id = ?`); err != nil {
+		return
+	}
 
-// TODO decide on rest of API (eg Validate, Use, Get, etc)
+	if result, err = stmt.Exec(i.ID); err != nil {
+		return
+	}
+
+	if rowsAffected, err = result.RowsAffected(); err != nil {
+		return
+	}
+
+	if rowsAffected == 0 {
+		err = errors.New("no rows affected")
+	}
+
+	return
+}