One of the questions you need to consider are the requirements you set for folks to be eligible to take your mission. These are the various requirements that you can select.
Minimum Shares Owned
Maximum Shares Owned
Shares Owned for at Least
Member of Community
There is no charge to set the shareholder requirement. Setting the other requirements will cost you a bit. Many have suggested improved targeting for missions, and I believe that is coming.
Consider shareholder versus anyone versus non-shareholder. You might select shareholders to provide a nice reward for them. You may need something done that is a bit tougher; so shareholders are more likely to get it done; since they already have a relationship with you. Shareholders are less likely to steal; because you can sell/block them & cause pain. Non-shareholder will get more mission theft issues, but you will also get some new investors typically. It is also a great way to meet new folks. If you are looking for new investors, a buy me mission to non-shareholders can help.
I seldom set requirements for my missions; probably because I am just not smart enough. As I often say, use your goals to guide your strategy.