Unfortunately, I do not usually daven Shachris nor do I put on Tefilin. My school has a Mincha minyan everyday. Am I allowed to daven with them if I haven’t yet put on Tefilin/and or I don’t have Tefilin with me?
You may daven and should daven with them even though you did not yet put on tefillin. If you will put them on for mincha that is even better.
As a side point it is good that you asked this question, because it is important for you to know that it is imperative that you make every effort to put on tefillin every weekday (even if you are not going to daven shacharis), and you should make every effort to try to keep to it. ( You don’t want to be considered a “karkafta d’lo monach tefillin” – a person that didn’t put on tefillin). Even if you will only put them on for a minute or two, and say the beginning of shema and then take them off, it is very important (or course if you can daven Shemona Esrei with them that is even better). Even more important than davening shacharis! It is true that sometimes you might be busy etc. but your neshoma will thank you tremendously for making the effort every day.
Hashem should give you the resolve and strength to keep it up.