Pandas - Group by one column and aggregate other column to list

I have a dataframe that has multiple entries for users. These users can also be assigned to multiple ID's.

I would like to group by the users and then store a list of these ID's in another column as shown below:

I'd like to go from this:

df1 = pd.DataFrame({'USER': ['BOB','STEVE','PAUL','KEITH','STEVE','STEVE','BOB'],'ID':[1,2,3,4,5,6,7]})

To this. Only showing values if that user is attached to multiple ID's

groupby + map

u = df1.groupby("USER")["ID"].agg(list)
df1["MULTI_IDS"] = df1["USER"].map(u[u.str.len().ge(2)])

USER      ID  MULTI_IDS
0    BOB   1     [1, 7]
1  STEVE   2  [2, 5, 6]
2   PAUL   3        NaN
3  KEITH   4        NaN
4  STEVE   5  [2, 5, 6]
5  STEVE   6  [2, 5, 6]
6    BOB   7     [1, 7]