Using a different apple account as compared to the one used while initial setup

I have two iMacs, first I have a 22" 2012, with the one piece thin screen. Then I bought a 27" 2015 iMac, 32gb ram and i7 processor.

I am trying to open three accounts that we can log on to and another if a guest needs to do something. I created several different accounts. Sometimes when we start up we can see our pictures over each of the accounts. But most of the time our start-up screen has only one account. But it has no picture of either of us and we don't know how or what apple id and password available. We can not guess how to access the only one. We never know what account we are going to be presented with.

What am I doing wrong? I have been using Mac's for over 30 years, so it must be something very simple.

Thanks

I believe one apple account would be ok but we have a family account with The Apple Store and we share space for our photography and videography. I think I will try both... I can't loose....

Good Luck!

The apps cannot be transferred to someone else. The licensed copy has your Apple ID encoded in the DNA of the copy and only you can redownload it or update it, even if you have authorized it's use on someone else's Mac.

Late 2012 Mac mini, OS X El Capitan 10.11.5 beta 3;  Watch, 38 mm silver AL, Watch OS 2.1; iPad Air 2 & iPhone 6+, iOS 9.3.2 beta 3;  Apple Airport Express

We are using different apple accounts, we each have an appleID and we use different Facebook and Yahoo accounts.

Thanks

I made two different partitions on the hard drive - one for each of us, but we have a family account with Apple Store.