If you join a clan after the weekly reset, you will not be able to earn and therefore contribute XP to your new clan for that week. Once the XP table refreshes at weekly reset, you'll be able to.
EDIT: There also is a known issue with clan rosters not being displayed for PC in-game (https://www.bungie.net/en/Forums/Post/236455208)
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Other's joined this week with no issue and we aren't capped. It seems it may be because I was in a (single person) guild before i joined
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Sounds like you left your previous clan after Tuesday which would lock you out for the week.