Síguenos is a standard Spanish word. The verb seguir means “to follow.” Sigue is the affirmative tú command (second-person singular informal imperative) of seguir. Nos is a direct object pronoun that means “us.” With affirmative commands, pronouns are compounded with the imperative verb (that is, with the command). Therefore, síguenos means “follow us!” This appears frequently in social media contexts—for example, an advertisement for a business might say “¡Síguenos en Facebook, Instagram y Twitter!” (“Follow us on Facebook, Instagram, and Twitter!”)—which is probably why you could find it in a texting dictionary.
So why the accent with síguenos? Well, because sigue ends in a vowel, the accent lies on the second-to-last syllable, which is the same as the first syllable in this case. However, the rule is that if the word ends in a vowel, n, or s, the stress falls on the second-to-last syllable. Because siguenos (notice the lack of an accent) ends in an s, the stress would fall on the syllable gue. This, however, is incorrect, because whenever pronouns are compounded (as enclitics) with commands, participles, or infinitives, the stress must remain where it was originally without any attached pronoun. Therefore, the accent mark over the i in síguenos indicates that the stress should not be on the second-to-last syllable, but rather on the first syllable, where the stress was originally before the pronoun became attached to the command.
In short, the accent is added to keep the stress on the first syllable of siguenos after the pronoun is compounded with sigue, because otherwise the accent would fall on gue.
I recommend that you read about accent rules with pronouns and commands in Spanish here.