When you create groups using one of the automatic or manual techniques discussed in Creating Groups, NodeXL inserts the group information into the workbook's Groups and Group Vertices worksheets. One row gets inserted into the Groups worksheet for each group, and one row gets inserted into the Group Vertices worksheet for each grouped vertex.
You should let NodeXL manage the rows in both worksheets. Manually adding or removing rows in the Groups and Group Vertices worksheets is not necessary and is not recommended.