Multiselect some of the record

The best way to do this is probably to set up a new Action process, where the first form contains a selectable paging grid showing a list of the records the user can select from. Upon submitting the form, the process could capture an array of the record primary key ID's selected, and loop over a subprocess (once per ID) where the subprocess takes care of performing the work/update you require.

I've done this as mschmitt recommends above with success. Additionally, in one scenario I put the new action as a Related Action on the records - when the RA is opened, the grid is displayed with the selection defaulted to the record they were viewing, with others selectable as well. From there, processing depends on what you need to accomplish.

Thanks for your inputs.

Remember that the records and related actions are contextual and there is nothing at the records level as an action, it is at individual record level.
So what csteward has suggested is a good way to keeping the context by having the default as record from where the related action was triggered and allowing to select other records while you enter a process.

An addition to mschmitt has proposed would be to have an action with start form where the start form is nothing but the grid view of all the records (like on the records tab) and from there you select and proceed.
Why a start form? Because it won't create a process till you actually have selected records and started editing, thus no side affect till you have made the selection like you do on the records tab

Good suggestion ! thanks Nikhil.

Another approach is to select additional records whenever you trigger the Related Action from 'A' record. This way, you can use the related action to act as multi select and you can use a Record picker (Or a selection Grid) to allow the user to add additional records.

I think I lost you with the additional records concept .Can you explain a little on this ?

I suppose Sathya is suggesting what csteward has suggested.
To addition to what I suggested, you can even create a site with the action (with start form), thus the site page will directly show the record list because as soon as you go to that site the action start form will show up which you can configure to show the selectable record grid