When a user want to join a group by clicking on the 'Request group membership' link they get the option to provide a 'Request message' as shown in the screenshot below. This can be confusing and in some situations this message is totally unnecessary. But there is no option within the Organic Groups settings to disable the request message. Have no fear! I've found way!

1. Go to the 'Configuration' of your Drupal website.
2. Go to 'OG membership types'.
3. Here you will see all your OG membership types. Below you will see the 'Default' membership type which comes 'out of the box' with Organic Groups.
4. Click on 'manage fields' next to the Organic membership type where you want to remove the request message.
5. Here you can delete the 'Request message' field. 

Now the request message won't show up anymore when a user wants to join a group. Did this tutorial help you? Let me know in the comments by saying thanks.

Organic GroupsDrupal 7