Select the strokes you wish to group together (read about Selection here), then tap the “link” icon on the selection popup. This will group your strokes into a single object, which can then be selected as a single item instead of needing to reselect all of them (such as a character, landscape item or other object). To ungroup them, tap the link again.
Grouping strokes does not transform them into a single vector stroke (Concepts does not have this ability at this time). They remain separate yet grouped.
Any grouped object can be edited as a single entity (if you change the color / tool etc, the change will apply to all strokes in the selection). To edit individual elements of a grouped object, you must first ungroup them.
If you wish to make your own objects in the object library, we recommend grouping your strokes into a single object first, to make selecting and moving them about easier. You can always ungroup them with a tap later.