Do items like the Guardian Seal affect it?
No. The Guardian Seal (and other similar Covenant rings) are only used for PvP. The summoning system is also completely different - entirely independent of the Soapstones.
Does being part of [...] (a) convenant [...] affect the tier ranges for using White Sign Soapstones?
Again, no. The only difference being in a specific covenant makes is it makes you more likely to have been summoned by a member of a similar covenant, but this is nowhere near a guarantee, and as I mentioned before, makes no difference to the summoning tiers.
The only items that do affect the tier ranges are using different Soapstones -
For PvE (Coop):
- The Small Soapstone gives you a range of -4/+2
- The Regular Soapstone give you a range of -3/+1
- Using the Name-Engraved Ring (with either the Small or Regular Soapstone) give an additional -3/+3, but only if other players have chosen the same god.
- The Red Soapstone gives you a range of -5/+2
- The Dragon Eye gives you a range of -5/+5
- The Cracked Red Eye Orb gives you a range of -0/+4 (your tier and above only)
- The Cracked Blue Eye Orb gives you a range of -3/+3
- The Bell Keeper's Seal gives you a range of -1/+3
Post Note: I would suggest that outside of the systematic functions of summoning in-game, the best thing to do is be part of the Heirs of the Sun, due to the glowing yellow summon sign (players are generally attracted to it due to it's presence in memes and similar jokes throughout the internet). Praise the Sun!