We use the forking model for collaboration within our team of ~7 developers. We've been using this model for around 3 years and migrated over from github. The issue we have is that in collaboration, it is often useful when reviewing a pull request to have access to the user's fork. The two most common caes cases this is valuable in our organization is:
1) You want to fetch their fork so you can test and work with their code before it is upstream.
2) You want to see the raw file. We use pull request for design docs so we can comment in-line, but without access to the user's repo, you cannot view the file in markdown format.
However, in order to make this work for our team, we have to give access to our private forks to the other developers which put us at more than 5 users accessing our private repos. This means we must not only pay for an upgraded account for the group account, but for each individual. This means to get forking model to work we have to pay (7+1)*10 = $80/mo instead of just the $10/mo for a 10 user group. So to get the forking model to work for our needs, it would be N times more costly where N is the number of developers in your organization. I don't believe this is the intention of the pricing model, but I could be wrong :P.
The solution that would help us the best on this would be to allow read access on private repos to be granted without it being counted as part of the user limit. Would love to hear thoughts around how to get the forking model to work better under the current pricing limitations as well.