text: user6655: I am aware of this question Manually define clusters in Seurat and determine marker genes that is similar but I couldn't make tit work for my use case.
  So I have a single cell experiments and the clustering id not great I have a small groups of 6 cells (I know it is extremely small, but nonetheless I would like to make the most of it) that are clearly isolate in UMAP and display marker that I am interresting in.
  I managed to get those cells names in barcode (e.g.: AGCCAGCTCGCTTTAT).
  I would like to compute differential expression of those cells against all others groups (using FindMarkers or FindAllMarkers).
  I tried to manually define the cluster in:
  Seurat_obj@meta.data$seurat_clusters
  But using FindMarkers (it says this groups don't exist)
  then I try to modify
  s_19270BC@active.ident
  but then it could not found metadata attached to my newly define group.
  I am kind of stuck and any help is warmly welcome
  Code
  My R is quite rusty...
  names_ # a vector with the names of the cell I want as a cluster
  
  # my seurat object have 5 cluster 0..4. So I want to add a 6th clusters with id 5
  
  #First attempt manually modify the vector active.ident and cluster
  # they are both factor so we need to add a factor level
  levels(seurat_obj@active.ident) = c(levels(seurat_obj@active.ident), 5)
  levels(seurat_obj@meta.data$seurat_clusters) = c(levels(seurat_obj@meta.data$seurat_clusters), 5)
  # then modify them :
  seurat_obj@active.ident[which(row.names(seurat_obj@meta.data) %in%  names_)] = 5
  seurat_obj@meta.data$seurat_clusters[which(row.names(seurat_obj@meta.data) %in%  names_)] = 5
  
  FindMarkers(s_19270BC, ident.1 = 5)
   
  
  So This morning it works... I don't know what I have done yesterday...
  EDIT:
  I added the Code I used
  
  user3541: It would have been better if you have shared your code as well as the error messages you got format these as such.
  As far as I can understand, your problem is that you failed to make your cluster as default ids. For this you can use:
  seurat_obj <- SetIdent(seurat_obj, id=seurat_obj@meta.data$column_of_interest).
  
  The cluster identities are stored in the @meta.data slot of your object.
  
original_text: 6655: <p>I am aware of this question <a href="https://bioinformatics.stackexchange.com/questions/4249/manually-define-clusters-in-seurat-and-determine-marker-genes">Manually define clusters in Seurat and determine marker genes</a> that is similar but I couldn't make tit work for my use case.</p>
  <p>So I have a single cell experiments and the clustering id not great I have a small groups of 6 cells (I know it is extremely small, but nonetheless I would like to make the most of it) that are clearly isolate in UMAP and display marker that I am interresting in.</p>
  <p>I managed to get those cells names in barcode (e.g.: AGCCAGCTCGCTTTAT).</p>
  <p>I would like to compute differential expression of those cells against all others groups (using FindMarkers or FindAllMarkers).</p>
  <p>I tried to manually define the cluster in:
  Seurat_obj@meta.data$seurat_clusters
  But using FindMarkers (it says this groups don't exist)</p>
  <p>then I try to modify
  s_19270BC@active.ident</p>
  <p>but then it could not found metadata attached to my newly define group.</p>
  <p>I am kind of stuck and any help is warmly welcome</p>
  <h1>Code</h1>
  <p>My R is quite rusty...</p>
  <pre><code>names_ # a vector with the names of the cell I want as a cluster
  
  # my seurat object have 5 cluster 0..4. So I want to add a 6th clusters with id 5
  
  #First attempt manually modify the vector active.ident and cluster
  # they are both factor so we need to add a factor level
  levels(seurat_obj@active.ident) = c(levels(seurat_obj@active.ident), 5)
  levels(seurat_obj@meta.data<span class="math-container">$seurat_clusters) = c(levels(seurat_obj@meta.data$</span>seurat_clusters), 5)
  # then modify them :
  seurat_obj@active.ident[which(row.names(seurat_obj@meta.data) %in%  names_)] = 5
  seurat_obj@meta.data$seurat_clusters[which(row.names(seurat_obj@meta.data) %in%  names_)] = 5
  
  FindMarkers(s_19270BC, ident.1 = 5)
   
  </code></pre>
  <p>So This morning it works... I don't know what I have done yesterday...</p>
  <p>EDIT:
  I added the Code I used</p>
   3541: <p>It would have been better if you have shared your code as well as the error messages you got format these as such.</p>
  
  <p>As far as I can understand, your problem is that you failed to make your cluster as default ids. For this you can use:</p>
  
  <pre><code>seurat_obj &lt;- SetIdent(seurat_obj, id=seurat_obj@meta.data$column_of_interest).
  </code></pre>
  
  <p>The cluster identities are stored in the <code>@meta.data</code> slot of your object.</p>
  