If at the point of registration I set members as 10, am I able to add the 11th ?

Yes you are allowed.

The system will update the entry you made to reflect the extra members.

You can also  adjust the group size manually.
The steps are as follows;
While on the dashboard, click on the menu icon as shown below.

This releases a settings menu.
On the menu, select edit group profile as shown below.

Click here to learn more, on how to set and adjust the group size.