Securing DB by using group accounts

Hello,

Fresh out of school, confused and often terrified, but did manage to create a
database that seems to be working nicely (probably because it did not go live yet).
I'm having issues setting up user accounts and permissions (never done any
secured dbs before). After playing with the wizard for an hour it seems to be
less clear than when I started out. I will be eternally grateful for any advice!!

There will be 2 departments using this DB, one of them (TC) needs read-only access,
and the other (MOS) read/update/insert, but I'm thinking no delete (leave it up to
their group admin). In order to keep things simple, the idea was to set up two
accounts (TC and MOS), plus MOS admin's account, instead of setting up individual
user accounts. The problem is, I don't know what the consequences are of multiple
users logging in under the same user name, whether it will impact the speed of the
tool, create some other unpleasant surprises, etc. Should I give up the whole idea?

Or I can create two group accounts with passwords? If yes, how would I do that? Is
wizard the best way to go about it (it's kind of confusing)? I had originally set up
TC (as a user account) on it, but then I messed it up when I was trying to add MOS.
So I do have Secured.mdw file created.

Thank you for your help and have a great day,

Sam.