Are you using the travel option to plan the getaway or just travelling to the lot directly? When I did the latter, it would not allow me certain sims to be assigned, but when I did the former I could choose everyone in the household plus friends etc.
If you are planning it, you should also make sure that the slots haven't all been auto-filled by the game preventing you from choosing further sims, eg:
If you look just underneath 'Assign sims to role' you can see it already filled 4 out of 6 spaces before I did any choosing. If you scroll down the sims you can see who already got assigned and un-select them so that you can choose someone you want.
Edit: Also, if the role has requirements (such as a skill level is needed) then only sims who meet the requirement will show up if that is in the settings. It's quite complicated!
Thanks I am using the plan travel option. I will check under the assign to role and see if that is what is preventing me. I thought I did but everyone is entitled to a senior moment. Right?
Well I played around for a bit but you are right. It is complicated at least so far. I may wait for some more youtube play videos until I get it right. LOL