Ability to create a segment or filter by selecting multiple client/contact names

I would like the ability to create a segment based on selection of multiple client or contact names without having to build a custom field first. This would be handy as we build and manage initiatives where participants may not share any factors except that they were selected for that initiative. Additionally, since it may be short-term or a rotating initiative, we don't want to build a custom field just to create a segment.