Users/Groups question

[vvor]

1 user creates/checkins/updates etc. this user is a member of the 'cvs'
group, as well as her own default group, as well as 'users'. User 1 changes
a file and it becomes owned by User1 default group.

Another user is unable to update "unable to create lock". User 2 is a member
of 'cvs' and his own default group, as well as 'users'. I tried making User
2 a member of the User1 default group. still get lock error. Furthermore, I
don't want User 2 to be a member of User 1 default group, and vice versa.

I fail to get users/groups. If 2 users are members of the same group the
most intuitive assumption is that they will both have the same access
without needing to account for invisible caveats and other system preemptive
rules that are inaccessible, unless those rules are reported to the user in
some manner.

I appear to be missing something basic. Any suggestions will be met with
godiva chocolates and rings of power.

vora