Newbie Data Relation Problem

I have two tables in a data set

Table Groups
GroupID (Key Int Identity field)
GroupName VarChar

Table Users
Firstnames
Surnames
GroupID (Int taken from GroupsTable.GroupID)

I am trying to set up a windows form which will allow me to display/edit the GroupName field in a combobox with a droplist of the available groups. The documentation says that I need to set up a data relation for this. When I do this it throws an exception when the form is run.

'Additional information: Failed to enable constraints. One or more rows contain values violating non-null, unique, or foreign-key constraints'

Of necessity the groupID field in the user table cannot be unique as there will be many users in each group.

How can I do this?

Thanks in anticipation
BrianHi Brian,

Pleaseview post 537644

Jason