First of all, I think that the other answers are fantastic. That said I'd just like to complement them with my own thoughts on the topic. (I'm going to paint a picture of a supportive introvert for the rogue in hopes of better communicating certain points. Remove supportive introvert and insert whatever motive you have for your character.)
Just because your character is a loner that doesn't mean that he can't watch out for the pack. Silent people are often INCREDIBLY observant, which often means that they get good at predicting the needs, desires, intentions, and actions of the group that they are with.
- Try to be observant and try anticipate the needs, desires, intentions, and actions of the party and it's members.
- Position your character as someone who can either fulfill the needs of the party or better yet be the character who enables the party to accomplish its goals collectively, and aid the sub-goals of party members.
- As a result you should be better able to endear you loner to the group as an essential member.
During play let the internal monologue that Brian Ballsrun-Stanton suggests show what your character thinks of his party and how he thinks his actions are supporting them. If you choose to open up your character at some point as okeefe's third point suggest then remember your character is
"...a quiet, contemplative rogue..."
if you don't want to break that introversion, but still want to let the other players know that you are thinking of them as you act, make the subtext that would naturally show up in room show up as actual text. Ask yourself questions like "what does my character do when he's looking out for his friends(or the closest things he has to friends)?" Whatever your answers are let that show up in his actions, but since you are online define your characters actions to the group, so that they know what you're trying to accomplish and why.
Example: The DM puts a group of thugs between you and your NPC bartender friend who helps you're party, the thugs are drunk and about to rough up your pal.
Player 1: "Let him go and we won't have a problem!"
Whispers to *Player 4* "I know where this is going." I figure that every hammer needs a good anvil. Player 1 might be able to hold them off for a while, but if I sneak behind them for a surprise attack she'll make short work of them.
Point being try to develop a rapport with your party, so that they don't just overlook you and the beautiful anvil you bring to their hammer. If you want the party to know that you're involved then you're going to have to go the extra mile to type up all of the subtext that you want to convey to them.