"Hi, I'm new to the group and just wanted to say 'HI'! I just love [insert subject of forum/community/group here] so much, and I'm glad to see others who agree!"What exactly does this add to the group, beyond clutter? Look, we know you like whatever, since you're here. Sad part is that these people usually have the least to say on the subject; their contributions end up mostly being "yeah, me too!" or "ooh, I hadn't seen that one!". Bring on an actual discussion, and they have no clue.
If said grouping ASKS you to introduce yourself, great, then actually introduce yourself: give some background, some substance, something that actually helps the rest of us get to know something about you. Otherwise, make your presence known by your participation, not your "hey, I'm a newbie" post.
Maybe I'm just being bitchy.