MemberID  RowNumber
123       1
124       2
125       3
211       4
212       5
214       6
320       7
321       8
322       9

MemberID  RowNumber  GroupNumber
123       1          122
124       2          122
125       3          122
211       4          207
212       5          207
214       6          208
320       7          313
321       8          313
322       9          313

declare @Members table (MemberID bigint);
insert @Members values (123), (124), (125), (211), (212), (214), (320), (321), (322);

with GroupedMembers as
(
    select
        M.MemberID,
        GroupNumber = M.MemberID - row_number() over (order by MemberID)
    from
        @Members M
)
select
    FromID = min(G.MemberID),
    ToID = max(G.MemberID)
from
    GroupedMembers G
group by
    G.GroupNumber
order by
    G.GroupNumber;

FromID  ToID
123     125
211     212
214     214
320     322