I need to add a search box to this form and may be in future I need to show user's email address as well, which place would be best for that ?
Place the search box above, right
Since the search applies to the list, place the search box and its label at the top of the list, on the right. The magnifying-glass icon is optional.
Here are some research-based guidelines for search that back up this answer in a round-about way—these guidelines are for web sites rather than applications.
A few other comments
- To avoid unnecessary clutter, you might want to hide the search box until the list is long enough to scroll.
- If you localize or translate the Add members and Search labels, and the text gets much longer, make sure the search box still fits with plenty of white space between the two.
- Consider removing the parenthesis from the (s) in the Add members label. The check boxes make it clear that users can select one or multiple.
- Consider using blue text for the New member command link.
- Switch the order of the Add and Cancel buttons if you intend to follow the very common Microsoft standard and Android standard.