Hi - You can have people from other groups work on a project in your group. Unless you've changed any of the project settings, if you are assigned to a task or issue on a project, you also get view access to the whole project. The only exception -ish to this is request queues. If you have in the Queue Details that only that project's group can see it, even if you've shared the project or assigned them an issue on the project, they will not see it in the Requests area. (They do see it in the Projects area).
Things I tell my people to be aware of when you have someone from another group in your project:
- If they are used to see certain statuses, that will not be the case here. They will only see statuses for the project's group so they might be a little different.
- They might not have access to any special views, filters, or groupings that you are used to using for your projects (since we tend to share these to a whole group).
My Print Studio agency (which is a separate group in Workfront) works on projects from 12 different groups and haven't had any issues.
Hope that helps. Let me know if there is a specific concern you are worried about.