How can I create a group of ASPNetUsers in SQL Server?

I need to create groups for users within an ASP.Net Identity framework.
The AspNetUsers table has a PK column labelled ID of type nvarchar(450).
The way I normally set up groups is to have a table with the useridindex column and a groupindex column, both comprising the primary key.
If I try this, I get a message that the index is limited to 900 bytes and won't allow me to do it.
Yes, I am new to this.
Thank you.