Users table and Group users table are history tables (they cannot be moved to archived tables cause that would mess up foreign keys related to those tables. If a group user made a post and left the group, the post should still exist etc)
Users
Username
Start Date
key (Username, Start Date)
Group Users
Group Name
Username
Start Date
Group Start Date
Primary Key (Group Name, Username, Group Start Date)
Foreign Key (Username, Start Date)
According to 2NF, any nonkey field cannot be a fact about a subset of a key. In this case, Username is part of a composite key and Start Date is a fact about Username, therefore this table is not in 2NF.
So would the fix be to include the start date as part of the key?
Not it isn’t. Start date cannot be determined by Username in your model because (Username, Start Date) is a key – the Username and Start Date together are needed to identify a row in the users table.