Begin with the assumtion that the multiple Workflows you want to make available to your users are Forms(Web) projects.
Create a master Forms(Web) project that identifies the user then determines which of the other Workflows the user has access to. For each of the target Workflows use a Link component on the master form as the mechanism used to launch it. The Link components would be made visible based upon the access you determined the user should have.
The Get Current User component can identify your user then you could perform an Active Directory query (Get User Details component) to identify the security groups the user is in.