Description:
Users and or groups are unable to edit diagrams even though they have Read/Write access set.

TIBCO.Nimbus uses hierarchical permissions set within the Manage Access Rights functionality. You will need to make sure you order the accounts and groups in the correct manner to allow access for the relevant groups.
For instance, if you had two user groups that had read only and one group that had read and write, you would need to make sure that the the group with the highest permissions set was at the top and any read only groups or users at the bottom of the list.

Users or groups only have read access when they are set as Read/Write to process maps.