Well, I suppose there are three options for you at the moment: You could start off your post with Sylvan in the group already, indicating that she had joined them at High Hrothgar and journeyed with them, but just hasn't said much or really introduced herself - if this is the case, I'd recommend you say it's because she's still unsure of joining properly yet, or something of the like.
A second option is your character goes to Ivarstead, but finds she is too late. There are currently four other characters in and around that area, so she could join with them as they search for Dovahfeyn.
A final option is meeting the group by pure coincidence, by yourself. Happen upon them in the wilderness, or just do a brief paragraph about how your character had missed them in Ivarstead - then do a timeskip forward one week in which she meets up with them now.
I'm not expecting tons of character development in your first post, but an insight to her personality would be great. Try to remember - I'm a total grammar Nazi, so keep the post nice and tidy please.