The following message appears when trying to add a user to your group and that user already has ArcWeb credentials:
"You are already a member of a different group. You can only be a member of one group at a time."
The ArcWeb Services product line has been retired. ESRI recommends that customers who wish to create Web applications or use services similar to those that were available in ArcWeb Services transition to the capabilities of ArcGIS Server and ArcGIS Online.
Each ArcWeb user can only be a member of one group. ArcWeb also cannot merge credentials from two user accounts.