Restrict members from showing up in @mentions

We need to restrict certain members of certain groups from showing up in @mentions.

Some use cases can include:

• Need to restrict @mentions to only members of the project (developers should not show up in non-software projects, customers should not show up in non-service desk projects they're a member of)
• Need to prevent certain groups from showing up in @mentions at all (contractors, customers/clients)

The big one that has come up is, service desk agents are able to @mention customers on internal projects and receive update notifications. The customer clicks on the issue but cannot vew it because they do not have JIRA application access.

Thanks!