Where are you using your grid? In a custom webpart or an ascx page template? Either rate, you'd have to code for it. Whatever dataset you retrieve your data from, you can specify in there a WHERE clause or SELECT rows WHERE a = a.
Regarding mass actions, you would also need to code this as well as there are none built-in with 5.5 that you can use out of the box. So maybe your unigrid has a checkbox in the first column, you can let users check/uncheck. Then on a button click you can iterate through all items and capture the ones checked and perform an action.