To make it possible to assign vendors in GroupShare from within BaccS UI, each contractor in BaccS database must be linked with corresponding GroupShare user. You have a number of options to do it.
If necessary contractors are missing click GroupShare - Import missing contractors button in the Contractors list.
The application will import from GroupShare all contractors which do not exist in BaccS database. Then, you can edit their details to add supported languages, specializations, prices, etc. This data will help to pickup correct vendor for your future project.
If you already have same vendor both in BaccS database and GroupShare, but they are still not linked, then you can select this contractor and click GroupShare - Link with GroupShare user on the ribbon. You will see a list of all your GroupShare users. Select one and click OK. Contractor and user will be linked, and you will be able to use it for GroupShare assignments.