Levels past 50. What Gives!!!!

I just reached level 50 and noticed I am no longer getting xp, how is it other players have levels higher, I saw someone at a level above 100, how is that possible, am I missing something please help.

push start to go to spartan hub go up one to career then pick specializations and select a spec it will let you level up more

You need to start your next specialization.

You need to select a Specialization. You have a choice between Operator and Wetworks. They in total will help you reach SR 70. Sometime in January or February , the other 6 specs will be released to everyone so you can reach SR-130

Thanks for the help!!!