You can lock yourself into a rank by not spending skill points to get past that rank, however you cannot stop your self from gaining skill points. Since missions are linked to your meta level (the skill point based level), you will keep getting new missions despite not being the appropriate rank for them.
That being said, if you want to stay a LT and play with your friends you can do so. You'll be stuck with the same ships as they, same equipment, and can play the same missions. It's really no different. All you have to do is not spend enough skill points to get to lt-commander, even if it's short by 100.
You can't stop your levelling, unless you stop playing. The next best thing to do is, when you enter a team with your friends, to pick one player as Squad Leader and use the option to match to the squad leaders level. This will increase (or reduce) your effective hit points and damage output. You can also choose to share quests (for example, for missions you already played but your team mate did not).
I want to keep my new guy as Lt, so I can play with any of my friends who come into the game. I almost have it, but I'm now at Lt 12, and all the missions/rewards are LC 2. Any suggestions?
he can level match your level.
You being a higher level and level matching missions will allow him to get a slight boost in EXP points because he is fighting mobs at a higher base level.
Or you can take a higher level character and have all his items and BOFF's, and level match him.
Your powers will get downgraded, but you will still have the boff's, to help you fly though the missions.