Hi Irvin,
To share a sub-site specifically for external users, please configure that site to make it has unique permissions from the rest of your sites. Then you can give permissions to external users only for this site.
Please refer to the steps as below:
1. Navigate to the sub-site, and then click Site Actions -> Site Settings ->Site permissions.
2. Click Stop Inheriting Permissions under Edit tab
3. Create a new group, add the external user account into this group and grant permissions to this group as you want. Or, you can grant the permissions to the external user directly.
When you invite an external user, the user shows up in the “Team Site Members” or “Team Site Visitors” group. Therefore, the group has permissions for entire site collection, so you need to remove the user from that group.
Thanks,
Nancy Xie