You can gain CP on a wide variety of mobs, either in a group or solo.
Escha/Reisenjima access is still a good idea to get for that, since those zones have a Cruor/Plasm/etc style currency called Silt you earn as a small percent of the XP you earn in those zones. While you do use it do buy Vorseals, temps, etc...there are also KIs that everyone in the group has to have in order to pop NMs there, which are purchased with Silt. Essentially, you will want to XP/CP in those zones at least a bit, in order to build up a nice stock of Silt.
Otherwise, once you're geared decently, a common destination for CP are the Gates zones in Adoulin, and often in the "map 2" areas that you need to have beaten the respective Wildskeeper Reive to enter. Offhand, these areas have either a reduced or removed party penalty, making them nice for grouping. Often, groups will target Apex mobs, which have relatively high HP, but also net abnormally high CP - a coordinated SC/MB will make short work of them.
Between Escha areas or non-Apex mobs in Gates areas, you can likely solo quite well too, though that varies by job.
That said, prior to really doing CP in earnest, you'd probably want to secure as many CP% bonuses as you can. BG Wiki's
Job Points article has a list. Also note the significant amount you get via Gifts - essentially, you get CP faster the more JPs you already have.