You can play MOST of the regular online missions by yourself in a closed session. Only some of the bigger missions require help and some of the bigger stuff like heists and etc require a full crew first.

You are still going to be grinding before you can do the missions you want as you have to open the mission givers first but to be fair the first parts of GTAO are pretty fun. its not until you are trying to grind out a few millions that it becomes an issue.