Converting cluster of Points to Polygons using QGIS

Convex hull - as mentioned by Kazuhito - is one option, but - depending on the cluster shape - you will get more appropriate polygons using concave hulls, for example implemented in ConcaveHull plugin.

You may be interested in Convex Hull which is in Processing | QGIS geoalgorithms | Vector geometry tools. There is Field option which can be used with Method Create convex hulls based on field.

Or from the menu Vector | Geoprocessing Tools | Convex Hull(s).

Many Thanks, Techie_Gus and underdark for information.