Permissions in FogBugz are not set directly on projects, wikis and discussion groups, but through the use of groups. A group is a set of permission levels for all of your normal and administrator FogBugz users (and optionally, community users). Every project is assigned one (and only one) group. Wikis and discussion groups can be open to all FogBugz users or be assigned a group. Each group can be reused many times to apply the same permissions in various places.
There are four basic ways to use groups:
Group by client
Working with a number of clients, you will probably want to give them access only to their projects. To do this, create a group for each client, giving all of that client's users permission in that group but no others. Next, edit each of the client's projects and set it to that group. You can also use groups to prevent your clients from seeing eachother. Learn how in Permissions under "Isolating users from one another".
Group by department
For each of your projects, various departments or roles may need different levels of access. You can achieve this by creating a group for each department or project type. For example, set all of your development projects to use the "Development" group. Setup this group to grant full access to programmers, testers and project managers, but read-only for customer Service representatives and salespeople. You can also setup an "Administrative" group which allows your management team full access, but none to all other users.
Set per-project permissions
You can create a one-to-one mapping of groups to projects to give each project custom permissions.
Setup groups by going to Admin | Groups. Then apply them by eding your projects, wikis and discussion groups.
See also the types of FogBugz users, Permissions, and managing a community wiki.