Add guest-account guidance to /join and 'Not a member yet' pages

app/main.py

@@ -600,6 +600,7 @@ async def join_page():
     <p>Thanks for joining the Inference Cooperative! To set up your account, connect your Open Collective account so we can verify your membership.</p>
     <a class="btn" href="/oauth/start">Connect Open Collective</a>
     <p class="note">You'll receive an account-setup email shortly after connecting.<br>Didn't get it? Contact <a href="mailto:info@inference.coop">info@inference.coop</a>.</p>
+    <p class="note">Contributed as a guest? <a href="https://opencollective.com/signin">Create an Open Collective account</a> using the same email you used to contribute, then return here and connect it.</p>
   </div>
 </body>
 </html>"""
@@ -686,6 +687,7 @@ async def oauth_callback(request: Request):
   <div class="card">
     <h1>Not a member yet</h1>
     <p>We couldn't find an active membership for your account. To join, contribute on our Open Collective page first, then return here to finish setup.</p>
+    <p class="note">Contributed as a guest? Make sure you've <a href="https://opencollective.com/signin">created an Open Collective account</a> with the same email you used to contribute, then try again.</p>
     <p class="note">Questions? Contact <a href="mailto:info@inference.coop">info@inference.coop</a>.</p>
   </div>
 </body>