As a general rule, we mostly send new survey invitations and reminders from Monday thru Friday. Reminders are also sent after hours and on weekends.
The invitations are sent to a set number of people (not the entire group) as we may have only been asked to get 500 completed surveys ( or 50 surveys, any number really).
If you've filled out your profile and you remain active then you'll be more likely to be chosen in the first targeted rounds.
We do random selections from the available member pool but there are often times when we can target only certain criteria so if you have your profile questions filled out that will help you get selected.
Also, this is market research, so each survey will have its own requirements on who should be invited so this limits the pool of invitee's. For example: If the survey is only for females 18-39 in rural WA, then males will definitely not be invited.