Co-Administrator can't see subscription in new Azure portal

I had this problem as well, in fact trying to see data via the resources.azure.com site was proving unfruitful. As it turns out the new portal has its own user management features that are more granular than the admin/co-admin of the old portal. Allow a subscription owner to make you a contributor or owner in the new portal and you should be able to see more data on the subscription via the new portal. To do that:

1. As the service administrator, go to the new portal
2. Browse to Subscriptions
3. Locate & select the subscription you need to add users into
4. In the subscription details blade, select settings
5. Under resource management, select users
6. You'll notice that even though you are a co-admin, you're not listed among the users.
7. Add the user and make them a contributor/owner

There's more information on Azure's role-based access control here: https://azure.microsoft.com/en-us/documentation/articles/role-based-access-control-configure/

The Azure Admin has changed so the 2 current answers are outdated. Follow the following steps:

1. Open the subscription, click on 'my permissions' then 'click here to view complete access details for this subscription'
2. In Access Control (IAM), Click on Role assignments, then click Add
3. Choose 'Add role assignment' then choose the role to be 'Contributor' and choose which user you want to add from the right hand pane , then click Save.
4. Make sure the user is listed as a contributor under 'Role Assignments'