There are two built-in roles specific to Azure OpenAI:
- Cognitive Services OpenAI Contributor
- Cognitive Services OpenAI User
To grant access, you can assign the 'Cognitive Services OpenAI User' role to your target user on the Azure OpenAI resource, and they will be able to use the Studio.
More detailed explanations of these roles are here:
https://learn.microsoft.com/en-us/azure/ai-services/openai/how-to/role-based-access-control