Those shares are for the filter which is chosen for the board. Only owner of the filter can add shares to the filter. If you push Edit Filter Query link and choose a filter which you own then you will be able to add shares.
A board is restricted to the users by shares of the filter on which the board is defined. In your case the board can be used by users who have access to mentioned projects.
You can read about restricting access to board here:
If you do not want the users of the projects use the board or the filter then do not share the filter with them
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