The short answer is late-coming members to a clan will not be able to achieve the same number of clan points as a founding member. Some missions won't award any/as many points as they would have if they'd been completed with the clan.
The long answer is that the first mission is 'search for ancient relics', which, if you join a clan late, has already been completed. This mission awarded group points only (no individual points). Since there is no mission to do individually on the first mission, you don't receive points for it. However, you still need to accept it and report it as done (I think), in order to move on to the next missions.
Any mission you complete on your own (things the clan has already marked as 'complete'), will only award individual points, not group points. Most of the missions will get you points, but really the bonuses are so small at this point that clan points aren't that beneficial.
The real benefit to completing clan missions (in my opinion), is access to the clan rooms. The storage room is my favorite place (250 kg of robber-free storage space!) that anyone in your clan has access to. So, if you have 2 chars in the same clan, they can easily share equipment/items without having to wait for them to drop on the market.
Hope that makes sense and have fun!
