CommunityGraphPlot without force layout

If you want to draw blob like CommunityGraphPlot:

iBlobs[style_, pts_, size_] :=
 Block[{epts},
  epts = Flatten[Tuples[CoordinateBounds[#, size]] & /@ pts, 1];
  {style, 
   FilledCurve@
    BSplineCurve[MeshPrimitives[ConvexHullMesh[epts], 1][[All, 1, 1]],
      SplineClosed -> True]}
  ]

{cliques, style} = {{{1, 2, 3}, {4, 5}, {4, 8}}, {Red, Green, Blue}};
vcoord = GraphEmbedding[g];

bsize = .08 Differences[MinMax[vcoord]][[1]];

bstyle = Directive[Opacity[.3], EdgeForm[#], #] & /@ style;

cpoints = vcoord[[VertexIndex[g, #] & /@ #]] & /@ cliques;

HighlightGraph[g, cliques, 
 Epilog -> 
  Activate[Thread[Inactive[iBlobs][bstyle, cpoints, bsize]]], 
 ImagePadding -> 20]

Update: Using GraphComputation`GraphCommunitiesPlotDump`generateBlobs, ... well, to generate blobs:

ClearAll[blobF, fC]
fC[coords_, size_: .04] := Module[{}, CommunityGraphPlot[Graph[{}], {}]; 
  FilledCurve @ BSplineCurve[GraphComputation`GraphCommunitiesPlotDump`generateBlobs[# &, 
    {coords}, size][[2, 1]], SplineClosed -> True]]


blobF[g_, cols_, coms_, size_: .04] := Thread[{cols, EdgeForm[{Gray, Thin}], Opacity[.25],
    fC[PropertyValue[{g, #}, VertexCoordinates] & /@ #, size] & /@ coms}];

{cliques,  colors} = {{{1, 2, 3}, {4, 5}, {6, 7, 8}}, {Red, Green , Blue}};
cg = CompleteGraph[8];
SetProperty[cg, {Epilog -> blobF[cg, colors, cliques], 
  VertexLabels -> "Name", ImagePadding -> 15}]

SeedRandom[1]
Row[{#, SetProperty[#, {Epilog -> blobF[#, colors, cliques], ImagePadding -> 15}]} &@
  RandomGraph[{20, 15}, ImageSize -> 300, VertexLabels -> "Name"]] 

Original answer:

{cliques,  colors} = {{{1, 2, 3}, {4, 5}, {6, 7, 8}}, {Red, Green , Blue}};
lines = Thread[{colors, CapForm["Round"], JoinForm["Round"], 
  Opacity[.5], AbsoluteThickness[20], 
  Line[PropertyValue[{cg, #}, VertexCoordinates] & /@ #] & /@ cliques}];

SetProperty[CompleteGraph[8], {Epilog -> lines, VertexLabels -> "Name", 
  ImagePadding -> 15}]

Or highlight Subgraphs:

cg = CompleteGraph[8];
Fold[HighlightGraph[##] &, cg, (Style[Subgraph[cg, #[[1]]], #[[2]], 
   CapForm["Round"], JoinForm["Round"], AbsoluteThickness[20]] & /@ 
 Transpose[{cliques, colors}])]